Abu Hurairah radiyallahu anhu reports, The Sahaabah asked, "O Messenger of Allah, you joke with us?" Rasoolullah sallallahu alaihe wasallam replied, "Yes, I do not say but the truth." (FR)
English reference (FR): Book 35, Hadith 227 (FR)
Arabic reference (FR): Book 36, Hadith 237 (FR)