Mbh 13.92.3a

"'"Agni said, 'Ye sires, when a Sraddha comes, we shall together eat the offerings made to us. If ye eat those offerings with me, ye shall then, without doubt, succeed in digesting them easily.' Hearing these words of the deity of fire, the Pitris became easy of heart. It is for this reason also that in making offerings at Sraddhas a share is first offered to the deity of fire, O king. If a portion of the offerings be first made to the deity of fire at a Sraddha, O prince of men, Rakshasas of regenerate origin cannot then do any injury to such a Sraddha.[414] Beholding the deity of fire at a Sraddha Rakshasas fly away from it. The ritual of the Sraddha is that the cake should first be offered to the (deceased) sire. Next, one should be offered to the grandsire. Next should one be offered to the great-grandsire. Even this is the ordinance in respect of the Sraddha. Over every cake that is offered, the offerer should, with concentrated attention, utter the Savitri Mantras. This other Mantra also should be uttered, viz., unto Soma who is fond of the Pitris. A woman that has become impure in consequence of the advent of her season, or one whose ears have been cut off, should not be allowed to remain where a Sraddha is being performed. Nor should a woman (for cooking the rice to be offered in the Sraddha) be brought from a Gotra other than that of the person who is performing the Sraddha.[414] While crossing river, one should offer oblations of water unto one's Pitris, naming them all. Indeed, when one comes upon a river one should gratify one's Pitris with oblations of water. Having offered oblations of water first unto the ancestors of one's own race, one should next offer such oblations to one's (deceased) friends and relatives. When one crosses a stream on a car unto which is yoked a couple of oxen of variegated hue, or from them that cross a stream on boats, the Pitris expect oblations of water.