St. Ambrose of Optina: If you do good, you must do it only for God!

If you do good, you must do it only for God. 

For this reason you must pay no attention to the ingratitude of people. Expect a reward not here, but from the Lord in heaven. If you expect it here — it will be in vain and you will endure deprivation.

St. Ambrose of Optina