四分律比丘含注戒本 2.53

2.53

If reciting together, whether once, twice, or thrice; whether instructing orally or instructing by writing; or if it is unclear; as well as with a non-human or animal—all are duṣkṛtas. If a teacher does not instruct by saying, "When I have finished speaking, you may speak," the teacher commits a duṣkṛta. As for non-offense: "When I have finished speaking, you speak"; one person finishes reciting and one person writes; or those of equal training reciting together. Or speaking in jest, speaking to oneself, or inadvertently speaking each other's words—none of these is an offense.