From the Introductory Section of the Atthasālinī, the Commentary to the Dhammasaṅgaṇī:
"The Mahāgosiṅga Sutta is even a stronger authority (to show that the Abhidhamma is the Buddha’s word). For therein when Sāriputta, the Generalissimo of the Law, approached the Teacher to inform him of the reciprocal questions and answers that took place between Mahāmoggallāna and himself, and told how the former had answered, (the Master said) [29]
“Brother Sāriputta, in the religion the talk of two bhikkhus on the Abhidhamma, each asking and answering the other without faltering, is in accord with the Dhamma. Now such a bhikkhu, brother Sāriputta, might enhance the beauty of the Gosiṅga Sāla Forest.”
The Teacher, far from saying that bhikkhus who knew Abhidhamma were outside his religion, lifted his drum-like neck and filling (with breath) his mouth, fraught as the full-moon with blessings, emitted his godlike voice congratulating Moggallāna thus: ‘Well done, well done, Sāriputta! One should answer rightly as Moggallāna has done; Moggallāna is indeed a preacher of the Dhamma.’
And tradition has it that those bhikkhus only who know Abhidhamma are true preachers of the Dhamma; the rest, though they speak on the Dhamma, are not preachers thereof. And why? They, in speaking on the Dhamma, confuse the different kinds of Kamma and of its results, the distinction between mind and matter, and the different kinds of states. The students of Abhidhamma do not thus get confused; hence a bhikkhu who knows Abhidhamma, whether he preaches the Dhamma or not, will be able to answer questions whenever asked. He alone, therefore, is a true preacher of the Dhamma. To this the Teacher referred when he approving said, ‘Moggallāna has well replied to questions.’
He who prohibits (the teaching of) Abhidhamma gives a blow to the Wheel of the Conqueror, denies omniscience, subverts the Teacher’s knowledge full of confidence, deceives the audience, obstructs the path of the Ariyas, manifests himself as advocating one of the eighteen causes of dissension in the Order, is capable of doing acts for which the doer is liable to be excommunicated, or admonished, or scorned (by the Order), and should be dismissed after the particular act of excommunication, admonition, or scorn, and reduced to living on scraps of food."
—The Expositor (Atthasālinī), trans. Maung Tin, PTS edition, Vol. I
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