Musnad Ahmad 2:12

Hassan narrated to us, he said: Ibn Lahi'ah narrated to us, Abu al-Aswad narrated to us, that he heard Muhammad ibn Abd al-Rahman ibn Labibah narrating from Abu Sinan al-Du'ali: That he entered upon Umar ibn al-Khattab while there were a group of the early Emigrants (Muhajirun) with him. Umar sent for a small box that had been brought to him from a fortress in Iraq. Inside it was a ring. One of his sons took it and put it in his mouth, so Umar snatched it from him, then Umar, may Allah be pleased with him, wept. Those who were with him said to him: "Why do you weep when Allah has granted you victory, caused you to prevail over your enemy, and brought comfort to your eyes?" Umar said: "I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: 'The world is not opened up for anyone except that Allah, the Mighty and Majestic, casts enmity and hatred among them until the Day of Resurrection.' And I fear that for this."