四分律行事鈔資持記 2.450

2.450

殺戒。（佛在毘舍離，諸比丘修不淨觀，厭身歎死；外道比丘難提，受雇行殺，居士驚怖，因制。）標名中。簡於非畜，不犯重故。或名大殺，簡後小故。

The precept against killing. (The Buddha was in Vaiśālī. The bhikṣus practiced the contemplation of impurity, loathed the body, and praised death; the non-Buddhist bhikṣu Nanda was hired to commit slaughter, terrifying the laypeople, whereupon the prohibition was instituted.) In designating the title: it excludes non-humans and animals, because they do not incur a grave offense. Or it is called the great killing, to distinguish it from the lesser [killing] later.