Abhayarājakumāra Sutta
It contains the episode of Prince Abhaya visiting the Buddha at Rājagaha and setting him the questions suggested by Nigaṇṭha Nāṭaputta:
“Would a Tathāgatha say anything unpleasant or disagreeable to others? If he did, how would he differ from ordinary men? If he did not, how was it that the Buddha spoke of Devadatta as a reprobate, a child of perdition, etc. — words which angered and upset Devadatta?”
The Buddha answered that the question needed qualification and, noticing that the prince was nursing his little boy, who lay in his lap, asked him what he would do if a pebble or a stick got into his mouth.
“I should pull it out even if the blood flowed.”
“Just so would a Buddha state unpleasant truths in due season if necessary and profitable.”
At the end of the discourse Abhaya accepts the Buddha as his Teacher (M.i.391 ff).