Musannaf Ibn Abi Shayba 18:137

Abu Bakr narrated to us, he said: Abdullah ibn Idris narrated to us, from Muhammad ibn Amara, from Abu Bakr ibn Amr ibn Hazm, who said: In the letter of the Messenger of Allah (may Allah bless him and grant him peace) to Amr ibn Hazm: "And for the eye, fifty [camels]."