Yazid bin Abdullah (may Allah have mercy on him) said: We were at a place called al-Mirbad when a man with dishevelled hair came, holding a piece of red leather in his hand. We said to him: It seems you are from the desert. He said: Yes. We said to him: Give us this piece of leather, and he gave it to us. We read what was in it, and it said: "This is from Muhammad (peace and blessings be upon him) to Banu Zuhayr bin Uqaysh: If you bear witness that there is no god but Allah and that Muhammad (peace and blessings be upon him) is the Messenger of Allah, establish the prayer, pay the zakah, give the one-fifth (khumus) of the spoils of war, and give the share of the Prophet (peace and blessings be upon him) and the share that is reserved specially for the ruler, then you are safe under the protection of Allah and His Messenger." We asked the man: Who wrote this? He said: The Messenger of Allah (peace and blessings be upon him).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13368