When the beginning of Surah al-Muzzammil was revealed, they used to stand in prayer for a length similar to their standing during the month of Ramadan, until its final portion was revealed. Between the revelation of its beginning and its end was a span of one year.
حدثنا أحمد بن محمد يعني المروزي حدثنا وكيع عن مسعر عن سماك الحنفي
لما نزلت أول المزمل كانوا يقومون نحوا من قيامهم في شهر رمضان حتى نزل آخرها وكان بين أولها وآخرها سنة
[Its isnad is sahih] Ibn Jareer Tabari narrated it in his Tafsir, 29/78, 79, from the hadith of Masaar, through this route.