Sathya Sai Baba says that He is the second of a triple incarnation. The first was Shirdi Sai Baba and the third will be Prema Sai Baba. Below, we give the information that is known about Prema Sai Baba, with references to the sources. Some of the sources are contradictory with regard to Prema Sai and when he will come on Earth.
 

The first mention of Prema Sai by Sathya Sai Baba appears to be in the discourse Shiva Shakthi,  6 July 1963,  In a conversation between Shiva, Shakthi, and Bharadwaja after Bharadwaja performed a ritual, Shiva said that

they would take human form and be born in the Bharadwaja lineage, thrice: Shiva alone as Shirdi Sai Baba, Shiva and Shakthi together at Puttaparthy as Sathya Sai Baba, and Shakthi alone as Prema Sai, later.

More information about Prema Sai

Sathya Sai Baba said in this discourse of 9 September 60 (Chapter 31 of Sathya Sai Speaks volume 1):

I will be in this mortal human form for 59 years [Editor: we now think that Swami meant lunar years] more and I shall certainly achieve the purpose of this avatar; do not doubt it. I will take My own time to carry out My Plan so far as you are concerned. I cannot hurry because you are hurrying.

I may sometimes wait until I can achieve ten things at one stroke; just as an engine is not used to haul one coach, but awaits until sufficient haulage in proportion to its capacity is ready. But My Word will never fail; it must happen as I will.

Various Sai devotees, in their writings, have mentioned Prema Sai, describing what was said in interviews and so on. Some are given below. However, some of the information appears to conflict. We suggest that, after reading these, we don't try to figure out which one is right and wrong but simply wait to see what happens. Events will unfold when they are supposed to, and in the way they are supposed to. Let us wait and see, and while waiting, delight in Swami's uncertainty!

We have heard often that Prema Sai will be born eight years after the passing of Sathya Sai Baba, which will take place when Sathya Sai Baba is 96. In "The Sai Trinity", by Dr. S.P. Ruhela (Taring paperbacks, Vikas Publishing House Pvt Ltd, New Delhi, 1997), the author states that Sathya Sai Baba has been disclosing small amounts of information to close devotees from time to time and states that Sai Baba has predicted that Prema Sai will be born in Karnataka about 8 years after He leaves this body.

Howard Murphet, in his book Sai Baba: Invitation to Glory (Chapter 4), says that

Finally, Sathya Sai states, there will be Prema Sai who, one year after the passing of the Sathya Sai form, will be born in Karnataka (the old Mysore State), at a place between Bangalore and the city of Mysore.

On page 16 of the book Living Divinity author Shakuntala Balu writes,

Sri Sathya Sai Baba has said that there will be one more Sai Avatara called Prema Sai. The third Sai will be born in Gunaparthi, a village in the Mandya district of Karnataka. Thus, Sri Sathya Sai Baba refers not only to his past, but also to the future form he will assume as Prema Sai.

In Sathya Sai Baba, The Embodiment of Love, by Peggy Mason and Ron Laing, the authors write about a discussion they had about Prema Sai.

We wondered if the next descent as Prema Sai, to come very quickly after the present form is vacated at the age of ninety six [Editor: now considered to be lunar years], would be female.

Baba replied, "No, male, in Mysore -- Karnataka." (Since 1972, the state of Mysore is called Karnataka.)

In Glimpses of the Divine: Working with the Teachings of Sai Baba" (1993), author Birgitte Rodriguez writes

In a more private talk to students in recent years, Baba told them that the father of His next incarnation, as Prema Sai, had been born in the southern Indian state of Karnataka. He also said that the body of Prema Sai was in the process of being formed.

Karunamba Ramamurthy (Kannamma) is the author of Sri Sathya Sai Anandadayi, Journey with Sai, copyright 2002 by the Convenor, Sri Sathya Sai Books & Publications Trust, Prasanthi Nilayam. Her book is written from notes she made in her diary starting in 1944. In Chaper 9, the author records what Swami said during the late 1940's to devotees in the old temple (mandir):

The present Avatar will last for 96 years, and, after a gap of one year, Prema Sai Avatar will assume human body and last for 75 years. ...

In the present yuga, the tenth Avatar, Kalki, has split into three aspects as the forms of Shirdi Sai, Sathya Sai, and Prema Sai. This has not happened before and can be seen only in this Kali yuga. Sai Avatars have not taken place in earlier Kali yugas, nor will they take place in future Kali yugas. During Shirdi Avatar, I was preparing the food ... In Sathya Sai Avatar, I am serving the food. In Prema Sai Avatar, everyone will enjoy the fruits of the divine meal.

The missions of the triple incarnations

In Spirit and the Mind, by Sam Sandweiss, the author recounts an extended interview given by Sathya Sai Baba to the Senior Editor, Mr. R.K. Karanjia, of Blitz new magazine in September 1976.

Question: Why had this task to be divided into three separate incarnations of the Shirdi, Sathya, and Prema Baba?

Baba: They are not separate. I have already mentioned the complete oneness of the three in the final objective of the mission. ... Their tasks and powers requisite to them differ according to the time, the situation, and the environment. But they belong to, and derive from, the same divine body (dharma swarup).

...

The previous Avatar, Shirdi Baba, laid the base for secular integration and gave mankind the message of duty as work. The mission of the present Avatar is to make everybody realize that the same God or divinity resides in everyone. People should respect, love, and help each other irrespective of color or creed. Thus, all work can become a way of worship. Finally, Prema Sai, the third Avathar, will promote the evnagel news that not only does God reside in everybody, but everybody is God. That will be the final wisdom that will enable every man and woman to go to God.

The three Avatars carry the triple message of work, worship, and wisdom.

Hislop's changing ring

In John Hislop's My Baba and I, page 55-56, Hislop writes about Swami materializing a ring for Hislop with Prema Sai on it. The image of Prema Sai on the ring actually changed its position over the years. Hislop writes,

The stone was a cameo of Prema Sai, the loving Lord of Creation, destined to appear on Earth a few years after the death of the Sathya Sai Body. It was a brownish stone, highly glazed, sculptured in profile, the bridge and length of the nose visible with a suggestion of the arch of the left eye. It was a noble head with shoulder-length hair, moustache, and beard; the head resting on, or emerging from, a lotus flower. His countenance was tranquil, peaceful, majestic.

Baba said, "He is only now in the process of birth, so I cannot show more of him. This is the first time he is shown to the world."

...

Wherever I go, devotees ask to see the ring. ... In a year or so, those devotees who had seen the ring sometime earlier said something surprising on seeing it again. They said, "It is turning. The whole nose can be seen now."

My reply was always, in truth, that I could see no change. But they said, "You see it every day and do not notice."

... One day, [in 1980], at a devotee meeting, I was showing the ring, and I looked at it carefully. It struck me, "It is different! It is changing!.

Now the entire nose is there and visible, whereas at first the entire nose was not visible -- or better to say the nose merged into the edge of the stone and did not appear to be fully visible. But now there is a space between the nose and the edge of the stone. Moreover, a portion of the left eye can be seen and also a portion of the left cheek.