Abu Dawud Book 13, Hadith Number 2993.
Chapter : Not known.
Narated By Yazid ibn Abdullah : We were at Mirbad. A man with dishevelled hair and holding a piece of red skin in his hand came.
We said: You appear to be a bedouin. He said: Yes. We said: Give us this piece of skin in your hand. He then gave it to us and we read it. It contained the text: “From Muhammad, Apostle of Allah (PBUH), to Banu Zuhayr ibn Uqaysh. If you bear witness that there is no god but Allah, and that Muhammad is the Apostle of Allah, offer prayer, pay zakat, pay the fifth from the booty, and the portion of the Prophet (PBUH) and his special portion (safi), you will be under by the protection of Allah and His Apostle.”
We then asked: Who wrote this document for you? He replied: The Apostle of Allah (PBUH).