SB 4.19.27

tam ṛtvijaḥ śakra-vadhābhisandhitaṁ
 vicakṣya duṣprekṣyam asahya-raṁhasam
nivārayām āsur aho mahā-mate
 na yujyate ’trānya-vadhaḥ pracoditāt
Synonyms: 
tam — King Pṛthu; ṛtvijaḥ — the priests; śakra-vadha — killing the King of heaven; abhisandhitam — thus preparing himself; vicakṣya — having observed; duṣprekṣyam — terrible to look at; asahya — unbearable; raṁhasam — whose velocity; nivārayām āsuḥ — they forbade; aho — oh; mahā-mate — O great soul; na — not; yujyate — is worthy for you; atra — in this sacrificial arena; anya — others; vadhaḥ — killing; pracoditāt — from being so directed in the scriptures.
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.