(147) بَابُ مَنْ لَعَنَ عَبْدَهُ فَأَعْتَقَهُ
'A'isha reported that Abu Bakr cursed one of his slaves and theProphet, may Allah bless him and grant him peace, said, "Abu Bakr! Thecursers and the true! No, by the Lord of the Ka'ba," and he repeated thattwo or three times. That very same day Abu Bakr freed one of his slaves.The Prophet, may Allah bless him and grant him peace, came and said, "Donot do that again (i.e. curse someone)."
Reference: Al-Adab Al-Mufrad 319
In-book reference: Book 15, Hadith 11
English translation: Book 15, Hadith 319