Musnad Ahmad 27:540

Hammad ibn Khalid narrated to us, from Ibn Abi Dhi’b, and Yazid said: Ibn Abi Dhi’b informed us, from al-Harith ibn ‘Abd al-Rahman, from Salim ibn ‘Abd Allah, from his father, who said: "The Messenger of Allah (peace and blessings of Allah be upon him) used to command us to be brief, yet he would lead us in prayer with [Surah] al-Saffat." Yazid said: "In the morning prayer."