Mbh 13.125.6a

"'"The Pitris said, 'Great is this question which thou hast asked. It involves a mystery and is fraught with wonder. We have been exceedingly gratified with thee, O ranger of the sky! The very deities and the Munis applaud acts done in honour of the Pitris. Even they do not know what the certain conclusions are of the ordinances in respect of the acts done in honour of the Pitris. Excepting the high souled, immortal, and excellent Markandeya, that learned Brahmana of great fame, who is ever devoted to the Pitris, none amongst them is conversant with the mysteries of the ordinances in respect of the Pitris. Having heard from the holy Vyasa what the end is of the three rice-balls offered at the Sraddha, as explained by the Pitris themselves in reply to the question of the celestial messenger, I shall explain the same to thee. Do thou hear, O monarch, what the conclusions are with respect to the ordinances about the Sraddha. Listen with rapt attention, O Bharata, to me as I explain what the end is of the three rice-balls. That rice-ball which goes into water is regarded as gratifying the deity of the moon. That deity, thus gratified, O thou of great intelligence, gratifies in return the other deities and the Pitris also with them. It has been laid down that the second rice-ball should be eaten by the spouse (of the man that performs the Sraddha). The Pitris, who are ever desirous of progeny, confer children on the woman of the house. Listen now to me as I tell thee what becomes of the rice-ball that is cast into the blazing fire. With that ball the Pitris are gratified and as the result thereof they grant the fruition of all wishes unto the person offering it. I have thus told thee everything about the end of the three rice-balls offered at the Sraddha and consigned to the three (viz., water, the spouse, and the fire). That Brahmana who becomes the Ritwik at a Sraddha constitutes himself, by that act, the Pitri of the person performing the Sraddha.