Musnad Ahmad 27:2011

He said: I read to my father, Hammad ibn Khalid al-Khayyat narrated to us, from Ibn Abi Dhi'b, from al-Harith ibn 'Abd al-Rahman, from Salim, from his father, who said: "The Messenger of Allah, peace and blessings of Allah be upon him, would indeed command us to lighten [the prayer], and he would indeed lead us in prayer with [Surah] al-Saffat."