Aisha (may Allah be pleased with her) narrates that the Prophet (peace and blessings be upon him) passed by Abu Bakr (may Allah be pleased with him) while he was cursing one of his slaves. The Prophet (peace and blessings be upon him) looked at him and said: "(Can) one who curses and the truthful (Siddiqeen) be together? By the Lord of the Ka'bah! Never." So Abu Bakr (may Allah be pleased with him) freed some of his slaves that day (as expiation), then came to the Prophet (peace and blessings be upon him) and said: I will not do so again. Imam Bayhaqi has narrated all five hadiths in Shu'ab al-Iman. Hasan, narrated by al-Bayhaqi in Shu'ab al-Iman.