From:
Theravāda Vinaya Theravāda Collection on Monastic Law
Mahāvibhaṅga The Great Analysis
Sekhiyakaṇḍa The chapter on training
Kabaḷavagga The subchapter on mouthfuls
43. Sakabaḷasikkhāpada 43. The training rule on with a mouthful
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 sakabaḷena mukhena byāharanti …pe…. At that time the monks from the group of six spoke with food in their mouths. …
“Na sakabaḷena mukhena byāharissāmīti sikkhā karaṇīyā”ti. “‘I will not speak with food in my mouth,’ this is how you should train.”
Na sakabaḷena mukhena byāharitabbaṁ. One should not speak with food in one’s mouth.
Yo anādariyaṁ paṭicca sakabaḷena mukhena byāharati, āpatti dukkaṭassa. If a monk, out of disrespect, speaks with food in his mouth, 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.
Tatiyasikkhāpadaṁ niṭṭhitaṁ. The third training rule is finished.