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." (RU)
English reference (RU): Book 35, Hadith 227 (RU)
Arabic reference (RU): Book 36, Hadith 237 (RU)