四分律比丘含注戒本 2.714

2.714

If he does not hear anything, it is a duṣkṛta. If he makes a preparatory effort wanting to go but does not go, or agrees with someone to go but does not go, it is in all cases a duṣkṛta. If two people are speaking together in a dark place, speaking together in a hidden place, or speaking together walking ahead, and one does not snap one's fingers or cough to alert them, it is in all cases a duṣkṛta. Bhikṣuṇīs commit the same offense as bhikṣus. As for non-offenses: if an unprofitable, unlawful karman is being performed and he listens because he wishes to know about it [to prevent it].