Musannaf Ibn Abi Shayba 23:1

Isma'il ibn 'Ulayyah narrated to us, from Abu Hayyan, from Abu Zur'ah, from Abu Hurayrah, who said: The Messenger of Allah (may Allah bless him and grant him peace) was one day appearing before the people, when a man came to him and said: "O Messenger of Allah, what is faith (iman)?" He said: "Faith is that you believe in Allah, His angels, His books, the meeting with Him, His messengers, and that you believe in the final resurrection." He said: "O Messenger of Allah, what is Islam?" He said: "That you worship Allah and do not associate anything with Him, establish the prescribed prayer, give the obligatory zakat, and fast Ramadan." He said: "O Messenger of Allah, what is excellence (ihsan)?" He said: "That you worship Allah as if you see Him, for if you do not see Him, He indeed sees you."