Sahih Muslim Book 4, Hadith Number 1309.

Chapter : Preference for saying the ‘Asr prayer at the commencement of the prescribed time.

‘Ali reported: When it was the day (of the Battle) of Ahzab, the Messenger of Allah (may peace be upon him) said: May Allah fill their graves and houses with fire, as they detained us and diverted us from the middle prayer, till the sun set.