Musnad Ahmad 938:34

Yahya ibn Sa'id narrated to us, from Hisham, who said: My father informed me that Zayd ibn Thabit, or Abu Ayyub, said to Marwan: "Did I not see you shorten the two prostrations (rak'ahs) of the Maghrib prayer? I saw the Prophet, peace and blessings of Allah be upon him, reciting Al-A'raf in them."