四分比丘戒本疏 2.100

2.100

第七十七、比丘共鬪諍者，四諍所攝共鬪也。聽而聞聲，即犯提罪。准律，若二人在闇地語，當彈指磬咳驚之，不者吉羅。二人前行亦爾。

Seventy-seventh, 'bhikṣus quarrelling and disputing together' refers to mutual fighting subsumed under the four types of disputes (adhikaraṇa). Eavesdropping and hearing their voices immediately constitutes a prāyaścittika offence. According to the vinaya, if two people are speaking in a dark place, one must snap one's fingers or cough to alert them; if not, it is a duṣkṛta. The same applies to walking behind two people.