Musnad Ahmad 938:71

'Abd al-Razzaq and Ibn Bakr narrated to us, they said: Ibn Jurayj informed us, he said: I heard 'Abd Allah ibn Abi Mulaykah narrating, he says: 'Urwah ibn al-Zubayr informed me that Marwan informed him, he said: Zayd ibn Thabit said to me: "What is the matter with you that you recite the short chapters of the Mufassal in the Maghrib prayer? The Messenger of Allah (peace and blessings of Allah be upon him) used to recite the longer of the two long chapters in the Maghrib prayer." I said to 'Urwah: "What is the longer of the two long chapters?" He said: "Al-A'raf."