Mbh 14.91.2a

"Vaisampayana said, 'Do thou listen to me, O king, as I discourse to thee duly, O Bharata, on the excellent ordinances relating to sacrifice and the fruits also, O ruler of men, that sacrifice yields. Formerly, on one occasion Sakra performed a particular sacrifice. While the limbs of the sacrifice were spread out, the Ritwijas became busy in accomplishing the diverse rites ordained in the scriptures. The pourer of libations, possessed of every qualification, became engaged in pouring libations of clarified butter. The great Rishis were seated around. The deities were summoned one by one by contented Brahmanas of great learning uttering scriptural Mantras in sweet voices. Those foremost of Adhwaryyus, not fatigued with what they did, recited the Mantras of the Yajurveda in soft accents. The time came for slaughtering the animals. When the animals selected for sacrifice were seized, the great Rishis, O king, felt compassion for them. Beholding that the animals had all become cheerless, those Rishis, endued with wealth of penances, approached Sakra and said unto him, "This method of sacrifice is not auspicious. Desirous of acquiring great merit as thou art, this is verily an indication of thy unacquaintance with sacrifice. O Purandara, animals have not been ordained to be slaughtered in sacrifices. O puissant one, these preparations of thine are destructive of merit. This sacrifice is not consistent with righteousness. The destruction of creatures can never be said to be an act of righteousness. If thou wishest it, let thy priests perform thy sacrifice according to the Agama. By performing a sacrifice according to the (true import of the) scriptural ordinances, great will be the merit achieved by thee. O thou of a hundred eyes, do thou perform the sacrifice with seeds of grain that have been kept for three years. Even this, O Sakra, would be fraught with great righteousness and productive of fruits of high efficacy."