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Narrated Qais: Bilal said to Abu Bakr, "If you have bought me for yourself then keep me (for yourself), but if you have bought me for Allah's Sake, then leave me for Allah's Work."
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا ابْنُ نُمَيْرٍ، عَنْ مُحَمَّدِ بْنِ عُبَيْدٍ، حَدَّثَنَا إِسْمَاعِيلُ، عَنْ قَيْسٍ، أَنَّ بِلاَلاً، قَالَ لأَبِي بَكْرٍ إِنْ كُنْتَ إِنَّمَا اشْتَرَيْتَنِي لِنَفْسِكَ فَأَمْسِكْنِي، وَإِنْ كُنْتَ إِنَّمَا اشْتَرَيْتَنِي لِلَّهِ فَدَعْنِي وَعَمَلَ اللَّهِ.
sunnah.com