Musnad Ahmad 26:545

Yahya narrated to us, Shu'bah narrated to us, Abu Ishaq narrated to me, from Abu al-Ahwas, from Abdullah, who said: Indeed, Muhammad, may Allah bless him and grant him peace, narrated to us: "A man lies until he is recorded with Allah as a liar, and a man speaks the truth until he is recorded with Allah as a truthful one (siddiq)."