Mbh 13.91.2b

Towards the feet of those Brahmanas engaged in eating the food that was served unto them a number of Kusa blades was spread out on the seats they occupied, with the top ends of the blades directed towards the south. With a pure body and mind and with concentrated attention, Nimi, having placed those blades of sacred grass in the way indicated, offered cakes of rice unto his dead son, uttering his name and family. Having done this, that foremost of Munis became filled with regret at the idea of having achieved an act that had not (to his knowledge) been laid down in any of the scriptures. Indeed, filled with regret he began to think of what he had done.[412] 'Never done before by the Munis, alas, what have I done! How shall I (for having done an act that has not been ordained) avoid being cursed by the Brahmanas (as an introducer of strange rites)?' He then thought of the original progenitor of his race. As soon as he was thought of, Atri endued with wealth of penances came there. Beholding him exceedingly afflicted with grief on account of the death of his son, the immortal Atri comforted him with agreeable counsels. He said unto him, 'O Muni, this rite that thou hast conceived, is a sacrifice in honour of the Pitris. Let no fear be thine, O thou that art possessed of the wealth of asceticism! The Grandsire Brahman himself, in days of old, laid it down! This rite that thou hast conceived has been ordained by the Self-born himself. Who else than the Self-born could ordain this ritual in Sraddhas? I shall presently tell thee, O son, the excellent ordinance laid down in respect of Sraddhas. Ordained by the Self-born himself, O son, do thou follow it. Listen to me first! Having first performed the Karana on the sacred fire with the aid of Mantras, O thou that art possessed of wealth of penances, one should always pour libations next unto the deity of fire, and Soma, and Varuna.