Musnad Ahmad 937:168

Yazid narrated to us, Muhammad ibn Ishaq narrated to us, from Makhul, from Ghudayf ibn al-Harith, a man from Ayla, who said: I passed by Umar ibn al-Khattab, and he said: "An excellent youth." Then a man from among those who were with him followed me and said: "O son of my brother, supplicate to Allah for me for good." I said: "And who are you, may Allah have mercy on you?" He said: "I am Abu Dharr, the companion of the Messenger of Allah, peace and blessings of Allah be upon him." I said: "May Allah forgive you; you are more entitled to supplicate for me than I am for you." He said: "O son of my brother, I heard Umar ibn al-Khattab when you passed by him just now saying: 'An excellent youth,' and I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: 'Indeed, Allah has placed the truth upon the tongue of Umar; he speaks by it.'"