This story comes in Aitareya Brahmana
the Risis, called the "middle ones" (Gritsamada, Visvamitra, Vaniadeva, Atri, Bharadvfija, Vasistha, see Asv. Grihya Sutras, 3, 4), hold once a sacrificial session on the Sarasvati. Amongst them there sat Kavasa.
These (Risis) reproached him (that he had come among them) saying : " Thou art the son of a slave girl, we shall neither eat nor drink with thee." Having become angry, he ran to the Sarasvati, and obtained her favour by moans of this hymn (pra devatrd braUmane).
She followed him. These Risis then thought that he was guiltless. Turning to him, they said, "Risi ! adoration be to thee, do us no harm ! thou art the most excellent among us, for she (Sarasvati) follows thee."
They made him the manager of the sacrifice, and thus appeased his wrath. This is the importance of Kavasa, and he it was who made that hymn known.
The mother is known as a slave girl, is the Father of Sage Kavasa mentioned? Who was the father of Sage Kavasa?