At one time when the Buddha was staying at Kosambī in Ghosita’s Monastery, a brahmin prepared a milk drink for the Sangha. The monks slurped while drinking the milk. A monk who was previously an entertainer made a joke of it, saying, “It’s as if the whole Sangha is cooled.” The monks of few desires complained and criticized him, “How could a monk joke about the Sangha?” … “Is it true, monk, that you did this?” “It’s true, sir.” The Buddha rebuked him … “Foolish man, how could you do this? This will affect people’s confidence …” After rebuking him … he gave a teaching and addressed the monks: “Monks, you should not joke about the Buddha, the Teaching, or the Sangha. If you do, you commit an offense of wrong conduct.” After rebuking that monk in many ways, the Buddha spoke in dispraise of being difficult to support … “And, monks, this training rule should be recited like this:

Final ruling ‘“I will not slurp while eating,” this is how you should train.’”

One should not slurp while eating. If a monk, out of disrespect, slurps while eating, he commits an offense of wrong conduct.

Non-offenses There is no offense:

if it is unintentional; if he is not mindful; if he does not know; if he is sick; if there is an emergency; if he is insane;

if he is the first offender. The first training rule is finished.

Theravāda Collection on Monastic Law The Great Analysis The chapter on training The subchapter on slurping Licking the hand

Origin story At one time the Buddha was staying at Sāvatthī in the Jeta Grove, Anāthapiṇḍika’s Monastery. At that time the monks from the group of six were licking their hands while eating. …

Final ruling “‘I will not lick my hand while eating,’ this is how you should train.”