Musnad Ahmad 1018:56

Muhammad and Hajjaj narrated to us, they said: Shu'bah narrated to us, from Qatadah, from Abd al-Rahman ibn al-Minhal or Ibn Maslamah, from his uncle. Hajjaj said: From Abd al-Rahman Abu al-Minhal ibn Maslamah al-Khuza'i, from his uncle, that the Prophet, peace and blessings of Allah be upon him, said to Aslam: "Fast today." They said: "We have already eaten." He said: "Fast the remainder of your day," meaning: the day of Ashura.