Musnad Abu Ya'la al-Mawsili 153:139

Amr ibn Muhammad al-Naqid narrated to us, [he said] Shababa ibn Sawwar and Yazid ibn Harun narrated to us, they said: Ibn Abi Dhi'b narrated to us, from al-Harith ibn Abd al-Rahman, from Salim ibn Abd Allah ibn Umar, from his father, he said: The Messenger of Allah (may Allah bless him and grant him peace) used to command us to lighten [the prayer], and he would lead us in the dawn prayer with [the Surah of] al-Saffat.