Sayyiduna Abu Hurayrah (may Allah be pleased with him) reports this saying of the Prophet (peace and blessings be upon him): "I do joke, but I say nothing except the truth." The people said: O Messenger of Allah, you joke with us? The Prophet (peace and blessings be upon him) said: I say nothing except the truth.
Imam al-Tabarani recorded this narration in al-Mu'jam al-Awsat, and its chain is hasan (good).