Mishkat al-Masabih 30 Fine Qualities كتاب المناقب 273

وَعَنْ قَيْسِ بْنِ أَبِي حَازِمٍ أَنَّ بِلَالًا قَالَ لِأَبِي بَكْرٍ: إِنْ كُنْتَ إِنَّمَا اشْتَرَيْتَنِي لِنَفْسِكَ فَأَمْسِكْنِي وَإِنْ كُنْتَ إِنَّمَا اشْتَرَيْتَنِي لِلَّهِ فَدَعْنِي وَعمل الله. رَوَاهُ البُخَارِيّ

Qais b. Abu Hazim told that Bilal said to Abu Bakr, “If you bought me only for your own sake keep me, but if you bought me only for God’s sake let me go and undertake God’s work.” Bukhari transmitted it.