Musannaf Ibn Abi Shayba 18:219

Abu Bakr narrated to us, he said: Ibn Idris narrated to us, from Muhammad ibn ‘Umarah, from Abu Bakr ibn ‘Amr ibn Hazm, he said: In the letter of the Messenger of Allah (may Allah bless him and grant him peace) to ‘Amr ibn Hazm, it was [written]: "For the hand, [the blood money is] fifty [camels]."