Sayyiduna Thawban (may Allah be pleased with him) narrates: The Messenger of Allah (peace and blessings be upon him) said: "O Allah, strengthen Islam through Umar bin al-Khattab," while Sayyiduna Umar (may Allah be pleased with him) (before accepting Islam) had been beating his sister at the start of the night as she was reciting: "Read in the name of your Lord who created" [Surah al-Alaq: 1], so much so that I thought he would kill her. Then he rose at the time of suhur and heard her voice reciting: "Read in the name of your Lord who created." He said: By Allah, this is neither poetry nor mumbling. Then he went until he came to the Messenger of Allah (peace and blessings be upon him) and found Sayyiduna Bilal (may Allah be pleased with him) at the door. He pushed the door, and Sayyiduna Bilal (may Allah be pleased with him) said: Who is this? He said: Umar bin al-Khattab. Sayyiduna Bilal (may Allah be pleased with him) said: Wait until I seek permission for you from the Messenger of Allah (peace and blessings be upon him). Sayyiduna Bilal (may Allah be pleased with him) said: O Messenger of Allah, Umar is at the door. The Messenger of Allah (peace and blessings be upon him) said: "If Allah intends good for Umar, He will bring him into the religion." Then he said to Sayyiduna Bilal (may Allah be pleased with him): Open the door. The Messenger of Allah (peace and blessings be upon him) took hold of his upper arms, shook him and said: "What do you want, and what have you come for?" Sayyiduna Umar (may Allah be pleased with him) said: Present to me that to which you invite. He (peace and blessings be upon him) said: "Bear witness that there is no god but Allah alone, who has no partner, and that Muhammad is His servant and His Messenger." So Sayyiduna Umar (may Allah be pleased with him) accepted Islam on the spot, and he (peace and blessings be upon him) said: "Go out."
Hadith Referenceالمعجم الكبير للطبراني / باب الثاء / 1415