It is narrated from Hazrat Aisha (may Allah be pleased with her) that I never saw the Prophet (peace and blessings be upon him) performing the night prayer while sitting. However, when the blessed body of the Prophet (peace and blessings be upon him) became heavy, he would perform as much prayer as Allah willed while sitting, and when thirty or forty verses of the surah remained, he would stand up, recite them, and then go into prostration.