佛說佛名經 19.18

19.18

“If a śikṣamāṇā, śrāmaṇera, or śrāmaṇerī repents of fundamental grave offenses, they should do so before four pure bhikṣus or bhikṣuṇīs according to the method above; after completing twenty-one days, you should know that they are pure, except if they lack sincerity.