Srimad-Bhagavatam: Canto 4 - Chapter 1 - Verse 27

Sanskrit:

तमृत्विज: शक्रवधाभिसन्धितंविचक्ष्य दुष्प्रेक्ष्यमसह्यरंहसम् । निवारयामासुरहो महामतेन युज्यतेऽत्रान्यवध: प्रचोदितात् ॥ २७ ॥

ITRANS:

tam ṛtvijaḥ śakra-vadhābhisandhitaṁvicakṣya duṣprekṣyam asahya-raṁhasamnivārayām āsur aho mahā-matena yujyate ’trānya-vadhaḥ pracoditāt

Translation:

When the priests and all the others saw Mahārāja Pṛthu very angry and prepared to kill Indra, they requested him: O great soul, do not kill him, for only sacrificial animals can be killed in a sacrifice. Such are the directions given by śāstra.

Purport:

Animal killing is intended for different purposes: it tests the proper pronunciation of Vedic mantras, and an animal being put into the sacrificial fire should come out with a new life. No one should ever be killed in a sacrifice meant for the satisfaction of Lord Viṣṇu. How then could Indra be killed when he is actually worshiped in the yajña and accepted as part and parcel of the Supreme Personality of Godhead? Therefore the priests requested King Pṛthu not to kill him.