"Then He created the openings between high skies and filled them with all classes of His angels. Some of them are in prostration and do not kneel up. Others in kneeling position and do not stand up. Some of them are in array and do not leave their position. Others are extolling Allah and do not get tired. The sleep of the eye or the slip of wit, or languor of the body or the effect of forgetfulness does not effect them."
ImamAli (a.s) Ibn e Abu Talib (a.s)
Ref: Nahj ul Balagha, Sermon # 1