Musnad Abu Ya'la al-Mawsili 247:4

Masruq ibn al-Marzuban al-Kufi narrated to us, Abu Bakr ibn Ayyash narrated to us, from al-A’mash, from Sa’id ibn ‘Abd Allah ibn Jurayj, from Abu Barzah, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "O assembly of those who have believed with their tongues, while faith has not entered their hearts: do not backbite the Muslims, and do not pursue their faults. For whoever pursues the faults of the Muslims, Allah will pursue his fault until He disgraces him in his own house."