score:4
It's in the commentary to verse 182 of the Dhammapada:
On seeing the Buddha, Erakapatta related to the Buddha how he had been a bhikkhu during the time of Kassapa Buddha, how he had accidentally caused a grass blade to be broken off while travelling in a boat, and how he had worried over that little offence for having failed to do the act of exoneration as prescribed, and finally how he was reborn as a naga.