the battle of al-Rayy

al-Mas'udi relates that al-Amin sent Ali ibn Isa ibn Mahan at the head of a large army toward Khurasan, and he turned aside to a district of the Rayy. Tahir ibn al-Husayn came against him with about four thousand horsemen. ²

al-Mas'udi says that Abbas ibn al-Layth struck a blow with both hands, and that Tahir was for this called Dhu al-Yaminayn ("Lord of the Two Hands"). A man known as Tahir ibn al-Raji slaughtered Ali, and they disputed over his ring and his head. ²

Tahir wrote to al-Fadl ibn Sahl that Ali's head was before him and his ring on his finger, and al-Ma'mun was pleased, and the caliphate was pledged to him. ²

The source gives no date here. It needs an additional source.