Musannaf Ibn Abi Shayba 34:210

Waki’ narrated to us, from Mis’ar, from Simak al-Hanafi, who said: I heard Ibn Abbas saying: When the beginning of [Surah] al-Muzzammil was revealed, they used to stand [in prayer] for a duration similar to their standing in the month of Ramadan, until its end was revealed. And between its beginning and its end was a year.