When the beginning of Surah al-Muzzammil was revealed, they would stand in prayer for a duration similar to their standing during the month of Ramadan, until the end of it was revealed. Between the revelation of its beginning and its end was a year.
لما نزلت أول المزمل كانوا يقومون نحوا من قيامهم في شهر رمضان حتى نزل آخرها، وكان بين أولها وآخرها سنة.
Abu Dawud (1305), Hakim (2/505), and Bayhaqi (2/500) all narrated it through various chains from Masaar, from Simak Hanafi, from Ibn Abbas, who said, then he related it. Its chain is hasan because of Simak Hanafi, for his hadith is hasan. Hakim said, "This is a hadith sahih in chain." His statement, "and recite the Quran with measured recitation" [Muzzammil 73:4] means a clear recitation, not a hurried one. It has come in the Sahih.