四分律比丘戒本 1.41

1.41

"If a bhikṣu sits alone together with a woman in a secluded place, a concealed place, a screened place, a place suitable for sexual intercourse, and speaks improper words, and an upāsikā whose words are trustworthy speaks of any one of the three rules—either pārājika, saṅghāvaśeṣa, or pāyattika—and the sitting bhikṣu himself says: 'I committed this offense,' he should be dealt with according to each one of the three rules: either pārājika, saṅghāvaśeṣa, or pāyattika. As spoken by the upāsikā whose words are trustworthy, this bhikṣu should be dealt with according to the Dharma. This is called an aniyata rule.