Musnad Abu Ya'la al-Mawsili 237:8

Abu al-Harith Surayj ibn Yunus narrated to us, Yahya ibn Zakariya ibn Abi Za'idah narrated to us, Mujalid narrated from 'Amir, who said: When the Messenger of Allah (peace be upon him) passed away and those who apostatized from the people apostatized, a group said: "We will pray, but we will not give Zakat." The people said to Abu Bakr: "Accept it from them." He said: "If they withheld from me a young she-goat, I would fight them." So he sent Khalid ibn al-Walid, and 'Adi ibn Hatim arrived with a thousand men from Tayy until he reached al-Yamamah. He said: The Banu 'Amir had killed the agents of the Messenger of Allah (peace be upon him) and burned them with fire. Abu Bakr wrote to Khalid to kill the Banu 'Amir and burn them with fire. He did so until the women cried out. Then he proceeded until he reached the water. They came out to him and said: "Allah is the Greatest, Allah is the Greatest. We bear witness that there is no god but Allah, and we bear witness that Muhammad is the Messenger of Allah." When he heard that, he refrained from them. Abu Bakr ordered him to march until he encamped at al-Hirah, then to proceed to al-Sham. When he encamped at al-Hirah, he wrote to the people of Persia, then said: "I would love not to depart until I frighten them." He raided them until he reached Sura, where he killed and took captives. Then he raided 'Ayn al-Tamr, where he killed and took captives. Then he proceeded to al-Sham. 'Amir said: Ibn Buqaylah brought out to me the letter of Khalid: "In the name of Allah, the Most Gracious, the Most Merciful. From Khalid ibn al-Walid to the Marzbans (governors) of the people of Persia. Peace be upon those who follow the guidance. I praise Allah, other than whom there is no god, with the praise that has severed your sanctity, dispersed your gathering, weakened your might, and stripped away your kingdom. When this letter of mine reaches you, accept the covenant (dhimmah) from me, pay the Jizyah to me, and send me hostages. Otherwise, by Him other than whom there is no god, I will surely bring against you a people who love death as you love life. Peace be upon those who follow the guidance."