He was born in a family of Sávatthi after his father had left home for the ascetic life. The mother, naming him Sánu, took him at the age of seven to the monks for ordination, thinking thus to ensure for him supreme happiness. He was known as Sánu (Sánu Sámanera) the Novice, and became a very learned teacher of the doctrine, practising the meditation of love (mettá), and was popular among gods and men.
His mother in a previous birth was a Yakkha. Later, Sánu lost his intellectual discernment and grew distraught and longed to go roaming. His former Yakkha mother seeing this, warned his human mother as described in the Sánu Sutta (q.v.). The latter was overwhelmed with grief, and, when Sánu visited her, he found her weeping. She told him that he was as good as dead in that he had rejected the Buddha's teaching and turned again to lower things, hence her sorrow. Sánu was filled with anguish, and, strengthening his insight, he soon won arahantship (ThagA.i.113f).
He is evidently identical with Udakadáyaka of the Apadána (Ap.i.205). In the past, he saw Siddhattha Buddha having his meal and brought him water for his hands and feet and face and mouth. Sixty one kappas ago he was a king, named Vimala.
The story of Sánu is given also in the Samyutta and Dhammapada Commentaries (SA.i.235ff.; DhA.iv.18ff), but the details differ. There, Sánu's human mother is portrayed as encouraging him to return to the lay life. His Yakkha-mother went to his human mother's home, where Sánu was waiting for a meal, took possession of his body, twisted his neck, and felled him to the ground, where he lay foaming at the mouth. Sánu's mother was filled with despair. The Yakkhiní then revealed herself and exhorted Sánu not to behave foolishly by disregarding the Buddha's teaching. When he regained his senses, his human mother, too, pointed out the disadvantages of household life. When he declared his intention of not returning to lay life, she fed him with choice food and gave him a set of three robes that he might receive the upasampadá ordination. He then sought the Buddha, who urged him to fresh and strenuous effort. Sánu was famous as a mighty teacher throughout Jambudípa. He lived to one hundred and twenty years.