It is narrated from Sayyiduna Jabir (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: I am also a human being, and I have taken a promise from my Lord that if any harsh words come out of my mouth regarding a Muslim, they will become a means of purification and reward for him.
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 14570