Someone comes to Muhammad from a great distance, yet without any sign of fatigue, and says: Muhammad, inform me about al-Islam. The Messenger of Allah replies: Al-Islam implies that you testify that there is no god but Allah and that Muhammad is the Messenger of Allah, and you establish prayer, pay zakat, observe the fast of Ramzan [Ramadan] and perform the pilgrimage. Later on, when the inquirer is gone, Muhammad tells Umar: He was Gabriel. He came to you to instruct you in matters of religion. This is the very first hadis narrated by Umar, the future KhalIfa, through several chains of narrators.