四分律比丘含注戒本 2.487

2.487

佛在舍衛。六群有緣在軍，觀諸鬪力，中有一人為箭所射，伴以衣舁，居士見怪。比丘聞告，佛因制戒。

The Buddha was in Śrāvastī. The group of six, having a reason to be in the army, watched the various combats; among them, one person was shot by an arrow and his companions carried him on a stretcher with robes, which the householders found strange. Bhikṣus heard of it and reported it, and the Buddha thereupon formulated the precept.