Mahabharata 1.93.5

1.93.5

yaṃ lebhe varuṇaḥ putraṃ purā bharatasattama |
vasiṣṭho nāma sa muniḥ khyāta āpava ityuta || 5 ||

"Vaisampayana continued, 'Thus addressed, the celestial daughter of Jahnu, Ganga, then replied unto the monarch, her husband, that bull amongst men, saying, 'O best of Bharata's race, he who was obtained as son by Varuna was called Vasishtha, the Muni who afterwards came to be known as Apava. He had his asylum on the breast of the king of mountains called Meru. The spot was sacred and abounded with birds and beasts. And there bloomed at all times of the year flowers of every season. And, O best of Bharata's race, that foremost of virtuous men, the son of Varuna, practised his ascetic penances in those woods abounding with sweet roots and water.