It is narrated from Marwan bin al-Hakam (may Allah have mercy on him): Zayd bin Thabit (may Allah be pleased with him) said to me: Why is it that you recite short surahs in Maghrib? Before you, I saw the Messenger of Allah (peace and blessings be upon him) reciting in the Maghrib prayer the longer of the two long surahs. I asked Urwah (may Allah have mercy on him): Which is the longer of the two long surahs? He said: Al-A'raf. Ibn Jurayj (may Allah have mercy on him) said: I asked Ibn Abi Mulaykah (may Allah have mercy on him): Which is the longer of the two long surahs? He said: Al-An'am and al-A'raf.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4036