Sayyiduna al-Aswad bin Sari' al-Tamimi (may Allah be pleased with him) said: I came to the Prophet of Allah (peace and blessings be upon him) and said: "O Prophet of Allah! I have composed a poem in which I have extolled Allah, the Mighty and Majestic, and praised you." He (peace and blessings be upon him) said: "What you have said in extolling Allah, recite it; and what you have said in praising me, leave it." So I began to recite, and a tall man with an aquiline nose came in. He (peace and blessings be upon him) said to me: "Stop." When he left, he (peace and blessings be upon him) said: "Recite." I began reciting again, and only a short while had passed when he came back, and he (peace and blessings be upon him) said to me: "Stop." When he left, he (peace and blessings be upon him) said: "Recite." I said: "O Prophet of Allah! Who is this man, that when he comes in you say 'Stop', and when he leaves you say 'Recite'?" He (peace and blessings be upon him) said: "This is Umar bin al-Khattab, and he has nothing to do with falsehood."
Hadith Referenceالمعجم الكبير للطبراني / باب الألف / 840