Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) says: We were sitting with the Messenger of Allah (peace and blessings be upon him) when a man appeared with extremely black hair and extremely white clothes. No trace of travel could be seen on him, and none of us knew him, until he sat down by the Messenger of Allah (peace and blessings be upon him), joined his knees to the knees of the Messenger of Allah (peace and blessings be upon him), placed his hands on his thighs, and said: "O Muhammad (peace and blessings be upon him)! Tell me about Islam: what is Islam?" The Messenger of Allah (peace and blessings be upon him) said: "Islam is that you bear witness that there is no deity except Allah and that Muhammad (peace and blessings be upon him) is His servant and Messenger, that you establish the prayer, give the zakah, fast in Ramadan, and perform Hajj to the House if you are able." The man said: You have spoken the truth. (He then mentioned the whole hadith.) Then he said: The Messenger of Allah (peace and blessings be upon him) said to me: "O Umar! Do you know who the questioner was?" I said: Allah and His Messenger (peace and blessings be upon him) know best. He (peace and blessings be upon him) said: "That was Jibril, who came to teach you your religion."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8610