Sahih Bukhari Volume 002, Book 014, Hadith Number 067.
Narated By Jabir bin 'Abdullah : On the day of the Khandaq Umar came, cursing the disbelievers of Quraish and said, "O Allah's Apostle! I have not offered the 'Asr prayer and the sun has set." The Prophet replied, "By Allah! I too, have not offered the prayer yet. "The Prophet then went to Buthan, performed ablution and performed the 'Asr prayer after the sun had set and then offered the Maghrib prayer after it."