Musannaf Ibn Abi Shayba 34:147

Yahya ibn Ya'la al-Aslami narrated to us, from Abdullah ibn al-Mu'ammal, from Abu al-Zubayr, from Jabir, who said: The beginning of Umar's Islam was—Umar said: My sister went into labor, so I was taken 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. I heard something the likes of which I had never heard before. 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 night or by day?" I feared that he might supplicate against me, so I said: "I bear witness that there is no god but Allah and that you are the Messenger of Allah." He said: "O Umar, keep it hidden." I said: "By the One who sent you with the truth, I shall surely proclaim it just as I proclaimed polytheism."