Musannaf Ibn Abi Shayba 26:404

Waki’ narrated to us, he said: Bastam bin Muslim narrated to us, from ‘Ata’ bin Abi Rabah, that Tariq bin al-Muraqqa’ emancipated a slave of his for the sake of Allah. Then he (the slave) died and left behind wealth. It was offered to his master, Tariq, and he said: "A thing I made for the sake of Allah, I will not return to it." So he wrote regarding that to ‘Umar. ‘Umar wrote: "Offer the wealth to Tariq; if he accepts it, then that is it. Otherwise, buy slaves with it and emancipate them." He said: So it reached fifteen heads.