Musannaf Ibn Abi Shayba 36:95

Abu Usama narrated to us, from Mujahid, who said: The Messenger of Allah (peace be upon him) wrote to my grandfather, and this is his letter which we have: In the name of Allah, the Most Gracious, the Most Merciful. From Muhammad, the Messenger of Allah, to ‘Umayr Dhu Maran and to those who have accepted Islam from Hamdan. Peace be upon you. I praise Allah to you, the One besides whom there is no god. To proceed: Your Islam has reached us upon our return from the land of the Romans. So rejoice, for Allah has guided you with His guidance. If you bear witness that there is no god but Allah and that Muhammad is the Messenger of Allah, and you establish prayer and give zakat, then you have the protection of Allah and the protection of Muhammad, the Messenger of Allah, over your blood, your wealth, and the land of Al-Bawn which you have accepted Islam upon—its plains, its mountains, its springs, and its pastures—without being wronged or restricted. Indeed, charity (sadaqah) is not lawful for Muhammad and the people of his house; it is only a zakat by which you purify your wealth for the poor of the Muslims. Malik ibn Murara al-Rahawi has guarded the unseen and conveyed the news, and I command you, O Dhu Maran, to treat him well, for he is looked upon with favor. Ali ibn Abi Talib wrote this. And peace be upon you, and may your Lord greet you.