Musnad Ahmad 938:66

Muhammad ibn Ja‘far narrated to us, Ibn Jurayj narrated to us, from Ibn Abi Mulaykah, ‘Urwah ibn al-Zubayr informed me, that Marwan informed him that Zayd ibn Thabit said to him: "What is the matter with me that I see you reciting short surahs during the Maghrib prayer? I have seen the Messenger of Allah (peace and blessings of Allah be upon him) reciting in it the longer of the two long ones." Ibn Abi Mulaykah said: "And what is the longer of the two long ones?" He said: "Al-A‘raf."