It is narrated from the Prophet of Islam (PBUH) who said: There are two good deeds which there are no other deeds better than them: 1) Believe in Allah (SWT) and 2) To be beneficial to other servants of God. And there are two bad deeds which no other ones are worse than them: 1) Polytheism (Shirk) and 2) Causing loss to servants of God [1].
[1] Mostadrakul Wasa’el, Vol. 12, Page 390.