An earlyish allusion to the gathering is in the Buddhavaṃsa’s chapter on Gotama Buddha:
ekosi sannipāto me, sāvakānaṃ mahesinaṃ,
aḍḍhateḷasasatānaṃ, bhikkhūnāsi samāgamo.
virocamāno vimalo, bhikkhusaṅghassa majjhago,
dadāmi patthitaṃ sabbaṃ, maṇīva sabbakāmado.
“I had only one assembly of disciples, great seers; it was a gathering of one thousand two hundred and fifty bhikkhus.
“Shining, stainless, in the midst of the bhikkhusaṅgha, like the jewel granting all desires, I gave everything that is aspired after.”
In the Buddhavaṃsa’s account of Maṅgala Buddha it’s said that all of the monks in his samāgama were of the ehi bhikkhu type; this detail is later transferred by the commentaries to the samāgamas of all Buddhas. As far as I know the other details are wholly commentarial.