What did Umar (saw) do during the Battle of the Trench that caused him to miss his `Asr prayer until the sun had set?
 Narrated Jabir:                     `Umar came cursing the disbelievers (of Quraish) on the day of Al-Khandaq (the battle of Trench) and said, "I could not offer the `Asr prayer till the sun had set. Then we went to Buthan and he offered the (`Asr) prayer after sunset and then he offered the Maghrib prayer.