As we sat one day with the Messenger of Allah (Allah bless him and give him peace), a man in pure white clothing and jet black hair came to us, without a trace of travelling upon him, though none of us knew him.
He sat down before the Prophet (Allah bless him and give him peace) bracing his knees against his, resting his hands on his legs, and said: "Muhammad, tell me about Islam." The Messenger of Allah (Allah bless him and give him peace) said: "Islam is to testify that there is no god but Allah and that Muhammad is the Messenger of Allah, and to perform the prayer, give zakat, fast in Ramadan, and perform the pilgrimage to the House if you can find a way."
He said: "You have spoken the truth," and we were surprised that he should ask and then confirm the answer. Then he said: "Tell me about true faith (iman)," and the Prophet (Allah bless him and give him peace) answered: "It is to believe in Allah, His angels, His inspired Books, His messengers, the Last Day, and in destiny, its good and evil."
"You have spoken the truth," he said, "Now tell me about the perfection of faith (ihsan)," and the Prophet (Allah bless him and give him peace) answered: "It is to worship Allah as if you see Him, and if you see Him not, He nevertheless sees you."
He said: "Now tell me about the Hour." The Prophet (Allah bless him and give him peace) answered: "The one who is asked about it does knows no more than the questioner."
He said: "Then tell me about its signs." The Prophet (Allah bless him and give him peace answered: "That a slave girl shall give birth to her mistress, and that you see barefoot, naked, destitute shepherds vying to build tall buildings."
Then the visitor left. I waited a long while, and the Prophet (Allah bless him and give him peace) said to me, "Do you know, 'Umar, who was the questioner?" and I replied, "Allah and His messenger know best." He said,
"It was Gabriel, who came to you to teach you your religion" (Sahih Muslim, 1.37: hadith 8).
This is a rigorously authenticated hadith, described by Imam Nawawi as one of the hadiths upon which the Islamic religion turns. The use of the word "religion" (din) the last words of it, "came to you to teach you your religion" entails that the religion of Islam is composed of the three fundamentals mentioned in the hadith: Islam, or external compliance with what Allah asks of us; Iman, or the belief in the unseen that the prophets have informed us of; and Ihsan, or to worship Allah as though one sees Him.
Sunni Islam, or Ahl al-Sunna wa'l-Jama'ah, understands the Islamic religion as it has been passed down in an unbroken chain of transmission from teacher to student from the time of the Prophet (Allah bless him and give him peace) until today. The way of Sunni Islam is to take the branch of Islam from living jurists who follow one of the four Sunni schools of fiqh: the Hanafi, Maliki, Shafi'i, and Hanbali schools; the branch of Iman from living scholars belonging to one of the two Sunni schools of 'aqida: the Ash'ari and Maturidi schools; and the branch of Ihsan from living masters of one of the many Sufi orders that have emerged over the centuries, such as the Qadiri, Naqshbandi, Shadhili, Chishti, and Rifa'i tariqas.