Al-Matalib al-'Aliyah 2917 — The signs of faith

Ibn Abi Rafi' narrates from a man who heard the Messenger of Allah (peace and blessings be upon him) speak when that man asked him (peace and blessings be upon him): "O Messenger of Allah! What is faith?" He (peace and blessings be upon him) said: "Faith is that you believe in Allah Almighty and His Messenger." He asked the same question a second time, and he (peace and blessings be upon him) gave the same answer again. Then when he repeated the question a third time, he (peace and blessings be upon him) asked: "Would you like me to tell you what the reality of faith and its clearest form is?" He said: "Yes! That is what I wanted." So he (peace and blessings be upon him) said: "The clear and manifest form of faith is that when you commit a wrong or wrong someone, whether your slave, your slave-woman or any ordinary Muslim, you give charity and fast (as expiation); and when you do a good deed, you feel joy and happiness at it (seeing it as Allah's enabling)."
Imam al-Harith bin Abi Usamah recorded this hadith in Musnad al-Harith (Bughyat al-Bahith).