The Book of Prayers (Kitab Al-Salat)
Muslim :: Book 4 : Hadith 1318
Jabir b. 'Abdullah reported that Umar b. al-Khattab had been cursing the pagans of the Quraish an the day (of the Battle) of Khandaq (Ditch). (He came to the Holy Prophet) and said: Messenger of Allah, by God, I could not say. the 'Asr prayer till the sun set. Upon this the Messenger (may peace be upon him) said: By Allah I, too, have not observed it. So we went to a valley. The Messenger of Allah (may peace be upon him) performed ablution and we too performed ablution, and then the Messenger of Allah (may peace be upon him) said the 'Asr prayer after the sun had set. and then said the evening prayer after it.