Sahih Bukhari, Chapter: Call to Prayers (Adhaan)

On the day of Al-Khandaq (the trench), 'Umar bin Al-Khattab went to the Prophet and said, "O Allah's Apostle! By Allah, I could not pray (the 'Asr) till the sun had set." 'Umar told this to the Prophet at the time when a fasting person had done Iftar (taken his meals). The Prophet then went to Buthan and I was with him. He performed ablution and offered the 'Asr prayer after the sun had set and then the Maghrib prayer.