Sahih Bukhari Volume 008, Book 075, Hadith Number 405.

Narated By 'Ali bin Abi Talib : We were in the company of the Prophet on the day (of the battle) of Al-Khandaq (the Trench). The Prophet said, "May Allah fill their (the infidels') graves and houses with fire, as they have kept us so busy that we could not offer the middle prayer till the sun had set; and that prayer was the 'Asr prayer."