四分比丘戒本疏 2.53

2.53

Twenty-first: according to the vinaya, the bhikṣuṇīs are to request an instructor every half-month; if there is a monk endowed with the ten virtues who receives the bhikṣuṇīs' request, the saṃgha must perform a ñattidutiya-kamma to appoint him to instruct the bhikṣuṇīs. Now, if one goes on one's own initiative to instruct the bhikṣuṇīs without having been appointed, one incurs a prāyaścittika. This meaning has already been briefly explained above in the preface to the precepts.