佛說目連問戒律中五百輕重事經 2.16

2.16

問：「鉢得合覆著壁上不？」答：「若巾裹得合淨處著，若囊或懸壁好，不得覆著壁上。昔六群比丘覆鉢壁上，墮地即破。佛因此制戒，自今已後不得覆鉢壁上，覆鉢壁上者犯捨。墮地者，犯捨墮。」

Question: "May a pātra be placed face down against a wall?"
Answer: "If wrapped in a cloth, it may be placed facing downward in a clean place. It is good if it is in a bag or hung on a wall; it must not be placed face down against a wall. Formerly, the ṣaḍvargika bhikṣus placed their pātras face down against a wall, and they fell to the ground and instantly broke. For this reason the Buddha instituted the rule: From this time forward, one must not place a pātra face down against a wall. Whoever places a pātra face down against a wall commits an offense of renunciation; if it falls to the ground, he commits a naiḥsargika-pāyattika offense."