Musnad Ahmad 937:252

Ya'la ibn 'Ubayd narrated to us, Muhammad—meaning Ibn Ishaq—narrated to us, from Makhul, from Ghudayf ibn al-Harith, who said: I passed by 'Umar while he was with a group of his companions. One of them caught up with me and said: "O young man, supplicate to Allah for me for good, may Allah bless you." I said: "And who are you, may Allah have mercy on you?" He said: "I am Abu Dharr." I said: "May Allah forgive you, you are more deserving of that." He said: "I heard 'Umar say: 'What an excellent young man,' and I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: 'Indeed, Allah has placed the truth upon the tongue of 'Umar; he speaks by it.'"