Aisha (ra) reported that “The Messenger of Allah (saw) used to strive more in worship during Ramadhan than he strove in any other time of the year; and he would devote himself more (in the worship of Allah) in the last ten nights of Ramadhan than he strove in the earlier part of the month (Sahih Muslim). Aisha (ra) also reported that the Prophet (saw) kept standing (in prayer) so long that the skin of his feet would crack. I asked him: “Why do you do this, while you have been forgiven of your former and latter sins?” He said, “Should I not be a grateful servant of Allah?” (Al-Bukhari and Muslim).