Sahih Bukhari Volume 001, Book 010, Hadith Number 570.
Narated By Jabir bin 'Abdullah : On the day of Al-Khandaq (the battle of trench.) 'Umar bin Al-Khattab came cursing the disbelievers of Quraish after the sun had set and said, "O Allah's Apostle I could not offer the 'Asr prayer till the sun had set." The Prophet said, "By Allah! I, too, have not prayed." So we turned towards Buthan, and the Prophet performed ablution and we too performed ablution and offered the 'Asr prayer after the sun had set, and then he offered the Maghrib prayer.