He was foremost among those who had the gift of varied and versatile discourse. AN.i.24 Once when Kumāra Kassapa was meditating in Andhavana a deva appeared before him, and asked him fifteen questions which only the Buddha could answer. This led to the preaching of the Vammika Sutta. MN.i.143ff.
Two verses of deep significance ascribed to Kumāra-Kassapa are found in the Theragāthā. Thag.201–202 The main example of his teaching is in the Pāyāsi Sutta, where he debated with a sceptic. DN.ii.316 This is the only sizable Buddhist scripture that also exists in a Jain version.
Kassapa’s upasampadā took place in his twentieth year. A doubt arose as to whether this was valid because, according to the rule, twenty years must be completed for upasampadā. The Buddha held that in reckoning the age the time spent in the mother’s womb could also be included. Vin.i.93