Musnad Ahmad 28:617

Mu'ammar ibn Sulayman al-Raqqi narrated to us, al-Hajjaj narrated to us, from 'Amr ibn Shu'ayb, from his father, from his grandfather, from the Messenger of Allah (peace and blessings of Allah be upon him), who said: "Whoever mutilates [someone] or burns [someone] with fire, he is free, and he is a freed slave of Allah and His Messenger." He said: A man who had been castrated, who was called Sandar, was brought [to the Prophet], and he emancipated him. Then he came to Abu Bakr after the death of the Messenger of Allah (peace and blessings of Allah be upon him), and he treated him well. Then he came to 'Umar after Abu Bakr, and he treated him well. Then, when he wanted to go out to Egypt, 'Umar wrote to 'Amr ibn al-'As regarding him: "Treat him well, or observe the will of the Messenger of Allah (peace and blessings of Allah be upon him) concerning him."