Musnad Ahmad 938:58

Sulayman ibn Dawud narrated to us, Abd al-Rahman ibn Abi al-Zinad informed us, from Hisham ibn Urwah, from his father, from Marwan ibn al-Hakam, who said: Zayd ibn Thabit said to me: "Did I not see you tonight shorten the recitation in the two prostrations [rak'ahs] of the Maghrib prayer?" "By the One in whose hand is my soul, the Messenger of Allah (peace and blessings of Allah be upon him) used to recite in them the two longest of the long [surahs]."