the prophet [pbuh] spent the whole night preceding the day of the battle in prayer and supplication. the muslim army, wearied with their long march, enjoyed sound and refreshing sleep, a mark of the divine favour and of the state of their undisturbed minds.

"(remember) when he covered you with a slumber as a security from him, and he caused rain to descend on you from the sky, to clean you thereby and to remove from you the rijz (whispering, evil suggestions, etc.) of satan, and to strengthen your hearts, and make your feet firm thereby." [al-qur'an 8:11]

that was friday night, ramadan 17th., the year 2 a.h.