What are Islam, Iman, and Ihsan, and where does Sufism fit?

Iman, Islam and ihsan are the three things the angel Jibril peace be upon him, known also as Gabriel, asked the Prophet ﷺ about, in the hadith upon which the reality of Tasawwuf is based.

It is narrated that the Prophet ﷺ appeared before the public one day when a man came to him and said, "O Messenger of Allah ﷺ, what is 'Iman' (faith)?" The Prophet ﷺ said, "Iman (Faith) is to believe in Allah, His Angels, His Books, meeting with Him, His messengers and to believe in the resurrection in the Hereafter."

He said, "O Messenger of Allah ﷺ, what is Islam?" The Prophet ﷺ said, "Islam is to worship Allah alone without associating anything with Him, to establish the prescribed prayer, to give obligatory charity and to fast the month of Ramadan."

He said, "What is 'Ihsan' (excellence)?" The Prophet ﷺ said, "Ihsan (Excellence) is to worship Allah as if you see Him. Verily, He sees you although you do not see Him."

The hadith continues, then the man left and the Prophet ﷺ asked his Companions to call him back, but they could not see him. The Prophet ﷺ said, "That was Gibrael peace be upon him who came to teach the people their religion." [Sahih al-Bukhari 50]

The use of deen, religion, in the last words of the hadith entails that the religion of Islam is composed of the three fundamentals mentioned in it: 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.

The level of Iman, or faith, is the Aqida, the tenets of faith; Islam is the Shariah, the Sacred Law, and its ancillary disciplines; and the level of ihsan, "to worship Allah as though you see Him," is Tasawwuf.