It was narrated that 'Abdullah bin 'Amr said:
"I heard the Messenger of Allah say: 'There is no one on earth, or under the sky, who speaks more truthfully than Abu Dharr.'" (ID)
English reference (ID): Vol. 1, Book 1, Hadith 156 (ID)
Arabic reference (ID): Book 1, Hadith 161 (ID)