It is narrated from Sayyiduna Sa'd bin Hisham (may Allah be pleased with him) that he came to Sayyidah Aishah (may Allah be pleased with her) and said: O Mother of the Believers! Tell me about the night prayer of the Prophet (peace and blessings be upon him). She said: Have you not read "O you wrapped in garments" [Surah al-Muzzammil: 1]? He says: I said: Of course! Sayyidah Aishah (may Allah be pleased with her) said: Allah Almighty made the night prayer obligatory at the beginning of the surah, so the Messenger of Allah (peace and blessings be upon him) and his Companions stood in prayer for a year, until their feet became swollen. Allah withheld the final part of the surah in the heaven for twelve months; then Allah sent down the lightening (of the obligation) at the end of the surah, and the night prayer became voluntary after having been obligatory.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1677