四分律比丘含注戒本 2.641

2.641

若比丘先入屋，彼人後入，若彼人先來，若俱入，隨脇著地，一一墮。

(If the bhikṣu enters the building first and that person enters later, or if that person comes first, or if they enter together, whenever a rib touches the ground, each one is an offense of falling.)