Mbh 1.131.4

“Vaisampayana said, ‘There dwelt at the source of the Ganga, a great sage named Bharadwaja, ceaselessly observing the most rigid vows. One day, of old, intending to celebrate the Agnihotra sacrifice he went along with many great Rishis to the Ganga to perform his ablutions. Arrived at the bank of the stream, he saw Ghritachi herself, that Apsara endued with youth and beauty, who had gone there a little before. With an expression of pride in her countenance, mixed with a voluptuous languor of attitude, the damsel rose from the water after her ablutions were over. And as she was gently treading on the bank, her attire which was loose became disordered. Seeing her attire disordered, the sage was smitten with burning desire. The next moment his vital fluid came out, in consequence of the violence of his emotion. The Rishi immediately held it in a vessel called a drona. Then, O king, Drona sprang from the fluid thus preserved in that vessel by the wise Bharadwaja. And the child thus born studied all the Vedas and their branches. Before now Bharadwaja of great prowess and the foremost of those possessing a knowledge of arms, had communicated to the illustrious Agnivesa, a knowledge of the weapon called Agneya. O foremost one of Bharata’s race, the Rishi (Agnivesa) sprung from fire now communicated the knowledge of that great weapon to Drona the son of his preceptor.