四分律比丘含注戒本 2.379

2.379

(Cases of non-offence: taking pure water or tooth-sticks. If unoffered clarified butter or oil is poured into the nostrils and comes out together with saliva, for the rest there is no offence. If, when a bhikṣu is begging for food, a bird drops food or the wind blows it into his alms bowl, he removes this food, up to what can be removed with a fingernail, and for the rest there is no offence.)