四分比丘戒本疏 2.13

2.13

第五、使非親尼浣故衣戒。浣染打三尼薩耆，下至一逕身著，即名為故也。

Fifth: the precept against having an unrelated bhikṣuṇī wash old robes. Washing, dyeing, and beating constitute three niḥsargika offences. Down to having been worn on the body just once, it is called "old."