四分律比丘含注戒本 2.223

2.223

The Buddha was in Śrāvastī. Kālodayin and the bhikṣuṇī Sthūlanandā were both handsome in appearance, and each harboured lustful thoughts, sitting together outside the gate. Householders censured them, comparing them to mandarin ducks. The bhikṣus heard this and informed [the Buddha], who instituted the rule on account of it.