From:
Theravāda Vinaya Theravāda Collection on Monastic Law
Mahāvibhaṅga The Great Analysis
Sekhiyakaṇḍa The chapter on training
Surusuruvagga The subchapter on slurping
54. Oṭṭhanillehakasikkhāpada 54. The training rule on licking the lips
Tena samayena buddho bhagavā sāvatthiyaṁ viharati jetavane anāthapiṇḍikassa ārāme. At one time the Buddha was staying at Sāvatthī in the Jeta Grove, Anāthapiṇḍika’s Monastery.
Tena kho pana samayena chabbaggiyā bhikkhū oṭṭhanillehakaṁ bhuñjanti …pe… At that time the monks from the group of six were licking their lips while eating. …
“Na oṭṭhanillehakaṁ bhuñjissāmīti sikkhā karaṇīyā”ti. “‘I will not lick my lips while eating,’ this is how you should train.”
Na oṭṭhanillehakaṁ bhuñjitabbaṁ. One should not lick one’s lips while eating.
Yo anādariyaṁ paṭicca oṭṭhanillehakaṁ bhuñjati, āpatti dukkaṭassa. If a monk, out of disrespect, licks his lips while eating, he commits an offense of wrong conduct.
Anāpatti—There is no offense:
asañcicca …pe… if it is unintentional;
ādikammikassāti. if he is the first offender.
Catutthasikkhāpadaṁ niṭṭhitaṁ. The fourth training rule is finished.