Sahih Adab al-Mufrad

It is narrated from Sayyidah Aisha (may Allah be pleased with her) that Sayyiduna Abu Bakr (may Allah be pleased with him) cursed one of his slaves, so the Prophet (peace and blessings of Allah be upon him) said: "O Abu Bakr! One who curses and also the Siddiq? Never!" By the Lord of the Ka'bah! The Prophet (peace and blessings of Allah be upon him) said this two or three times. Upon this, Sayyiduna Abu Bakr (may Allah be pleased with him) freed one of his slaves that day, and afterwards, he presented himself before the Prophet (peace and blessings of Allah be upon him) and said: I will not do this again.