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 made a covenant with my Lord that if any harsh words come from my mouth regarding a Muslim, it will be a means of purification for him.
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 15126