When Bhishma lay on his arrow bed, he told Arjuna about the 1000 names of Vishnu. These 1000 names constitute the Vishnu Sahasranama. Vishnu Sahasranama transcends philosophical differences, said V.S. Karunakarachariar in a discourse. It has commentaries by Sankara Bhagavadpada, Parasara Bhatta and Satyasanda Teertha. So, whether it is the Advaitic school or Visishtadvaitic school or the Dvaita school of philosophy, the Vishnu Sahasaranama was considered important enough for all three to write commentaries on it. PrasthAna traya refers to the three most important works of Vedanta — Upanishads, Brahma Sutra and Bhagavad Gita. Bhagavadpada wrote commentaries for these works, and also for Vishnu Sahasranamam.
How he came to write this commentary is interesting. He asked his disciple Padmapada to fetch a manuscript from the manuscript collection, and he also said Padmapada had to pick something randomly, with his eyes closed. The manuscript that turned up was the chapter titled Vishnu Sahasranama, in the Anusasana Parva of the Mahabharata. Bhagavadpada asked Padmapada to repeat the process, this time also with his eyes closed. The second time also, it was the Vishnu Sahsaranama that came up. Padmapada tried a third time, with the same result.
Another version says that Padmapada tried for a fourth time, and that a godly looking little girl, picked a manuscript for Padmapada. And the manuscript selected by her too turned out to be the Vishnu Sahasranama. When Bhagavapada came to know about this, he felt it was Goddess Parvathi Herself who had asked him to go ahead with a commentary on Vishnu Sahasranama. Taking it as a divine sign he wrote a commentary for the Vishnu Sahasranama.
