'A'isha reported that Abu Bakr cursed one of his slaves and the Prophet, may Allah bless him and grant him peace, said, "Abu Bakr! The cursers and the true! No, by the Lord of the Ka'ba," and he repeated that two 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, "Do not do that again (i.e. curse someone)."
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا أَحْمَدُ بْنُ يَعْقُوبَ قَالَ: حَدَّثَنِي يَزِيدُ بْنُ الْمِقْدَامِ بْنِ شُرَيْحٍ، عَنْ أَبِيهِ، عَنْ جَدِّهِ قَالَ: أَخْبَرَتْنِي عَائِشَةُ، أَنَّ أَبَا بَكْرٍ لَعَنَ بَعْضَ رَقِيقِهِ، فَقَالَ النَّبِيُّ صلى الله عليه وسلم: يَا أَبَا بَكْرٍ، اللَّعَّانِينَ وَالصِّدِّيقِينَ؟ كَلاَّ وَرَبِّ الْكَعْبَةِ، مَرَّتَيْنِ أَوْ ثَلاَثًا، فَأَعْتَقَ أَبُو بَكْرٍ يَوْمَئِذٍ بَعْضَ رَقِيقِهِ، ثُمَّ جَاءَ النَّبِيُّ صلى الله عليه وسلم فَقَالَ: لا أَعُودُ.
sunnah.com