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.'" (RU)
English reference (RU): Vol. 1, Book 1, Hadith 156 (RU)
Arabic reference (RU): Book 1, Hadith 161 (RU)