Musannaf Ibn Abi Shayba 36:65

Abu Bakr narrated to us, he said: Yahya bin Ya'la al-Aslami narrated to us, from Abdullah bin al-Mu'ammal, from Abu al-Zubayr, from Jabir, who said: The beginning of Umar's Islam was that he said: My sister's labor pains struck at night, so I was driven out of the house. I entered into the coverings of the Kaaba on a cold night. He said: Then the Prophet, may Allah bless him and grant him peace, came and entered the Hijr while wearing his sandals. He prayed as much as Allah willed, then he turned away. He said: I heard something the likes of which I had never heard. So I went out and followed him. He said: "Who is this?" I said: "Umar." He said: "O Umar, will you not leave me alone by day or by night?" He said: I feared that he would supplicate against me. He said: I said: "I bear witness that there is no god but Allah, and that you are the Messenger of Allah." He said: He said: "O Umar, keep it hidden." He said: I said: "By the One who sent you with the truth, I will surely proclaim it just as I proclaimed polytheism."