One place is in the Theragatha, Thag1.69
where Channa says:
I’ve entered the path to realize the deathless—
Maggaṃ papajjiṃ amatassa pattiyā
Although it seems this verse does not explicitly confirm his arahantship, the commentator Dhammapala says this verse was uttered both before and after Channa’s enlightenment.
See the athakata here:
he attained Arahantship but before long. Hence, has it been said in Apadāna.–
…Having, however, attained Arahantship, he uttered a stanza in order to make his joyous utterance releasing the speed of his zest, being well-satisfied with the bliss of emancipation…
- ”Having heard the highly delicious truth (dhamma) from the great, taught by the excellent omniscient knowledge (ñāṇa), I entered upon the right path (magga) for the attainment of immortality (amata); He is clever in the path towards security from fetters (yogakkhema). There, seized by ear, being attentive, and having realised in conformity with the ear-door. Dhammaṃ means the teaching (dhamma) of the four noble truths. Mahato means of the Blessed One. Indeed, the Blessed One is said to be “Mahā (great)” because of being endowed with such great, most lofty (uḷāra) qualities as moral precepts and so on, and also because of being praise worthy distinctively by the world together with the divine world. Mahāsamaṇo is to be construed thus: Whatever designation (samaññā) had sprung up for Him, this is a word in the ablative case (nissakka) “mahatodhammaṃ sutvāna, having heard the teaching (dhamma) from the great.” Mahārasaṃ means: lofty flavour because of being the giver of the flavour of emancipation. Sabbaññutañaṇavarena desitaṃ is to be construed thus:– He knows all; thus, He is Sabbaññu, the omniscient; the state of that is Sabbaññutā (the condition of being omniscient). The knowledge itself is excellent (vara); in other words: the excellent among kinds of knowledge (ñāṇesu): thus, ñāṇavaraṃ (the excellent knowledge): to him there is excellent knowledge of being omniscient; thus, sabbaññutañāṇa varo (He who has the excellent knowledge of omniscience); the Blessed One. Having heard the truth (dhamma) taught and pointed out either by that foremost knowledge, reckoned as omniscient knowledge, or by having become the instrumental case (karaṇa): thus, is the interpretation. Whatever, however, should be said here, that should be understood in the manner stated in the Commentary on Itivuttaka (known as) Paramatthadī-panī. Maggaṃ means the eightfold noble path (ariya magga). Pāpajjiṃ means I entered upon. Amatassa pattiyā means: I entered upon that which had become the approach (upāya) towards the attainment of nibbāna; thus, is the interpretation. So means: that Blessed One. Yogakkhemassapathassa kovido is to be construed thus:– whichever is the path of nibbāna where one is not haressed by four bonds (yoga) (of craving, existence, false views and ignorance), at that He is clever, well efficient there. This, indeed, is the meaning here:– having heard the teaching of the four noble truths, I entered upon the approach road towards the achievement of immortality; the path of entry was made by me; that Blessed One even was however, clever in the path of security from bonds (yoga) in every respect; He is clever in either the continuity of others (parasantāna) or the mind of others; thanks to whose arrangement (saṃvidhāna) I also entered upon the right path (magga). This self-same stanza itself was that of the thera’s exposition of his aññā(attainment)…
Hope that helps. Best wishes with your studies and your students!