Sunan an-Nasa'i #4146 · The Book of Distribution of Al-Fay'

"While I was with Mutarrif in Al-Mirbad, a man came in carrying a piece of leather and said: 'This was written to me by the Messenger of Allah. Is there anyone among you who can read?' I said: 'I can read.' And it was (a letter) from Muhammad the Prophet to Banu Zuhair bin Uqaish, who had testified to Lailah illallah, and that Muhammad is the Messenger of Allah, and had left the idolaters, and had agreed to give the Khumus from their spoils of the Prophet, and wheatever he chose for himself, so they became safe and secure by the covenant of Allah and His Messenger