Sahih BukhariVolume 5, Book 59, Number 437:

Narrated 'Ali:

On the day of Al-Khandaq (i.e. Trench), the Prophet said '(Let) Allah fill their (i.e. the infidels') houses and graves with fire just as they have prevented us from offering the Middle Prayer (i.e. 'Asr prayer) till the sun had set."