Sahih Bukhari Volume 1, Book 10, Hadith 641

حَدَّثَنَا أَبُو نُعَيْمٍ، قَالَ حَدَّثَنَا شَيْبَانُ، عَنْ يَحْيَى، قَالَ سَمِعْتُ أَبَا سَلَمَةَ، يَقُولُ أَخْبَرَنَا جَابِرُ بْنُ عَبْدِ اللَّهِ، أَنَّ النَّبِيَّ صلى الله عليه وسلم جَاءَهُ عُمَرُ بْنُ الْخَطَّابِ يَوْمَ الْخَنْدَقِ فَقَالَ يَا رَسُولَ اللَّهِ، وَاللَّهِ مَا كِدْتُ أَنْ أُصَلِّيَ حَتَّى كَادَتِ الشَّمْسُ تَغْرُبُ، وَذَلِكَ بَعْدَ مَا أَفْطَرَ الصَّائِمُ‏.‏ فَقَالَ النَّبِيُّ صلى الله عليه وسلم ‏"‏ وَاللَّهِ مَا صَلَّيْتُهَا ‏"‏ فَنَزَلَ النَّبِيُّ صلى الله عليه وسلم إِلَى بُطْحَانَ وَأَنَا مَعَهُ فَتَوَضَّأَ ثُمَّ صَلَّى ـ يَعْنِي الْعَصْرَ ـ بَعْدَ مَا غَرَبَتِ الشَّمْسُ، ثُمَّ صَلَّى بَعْدَهَا الْمَغْرِبَ‏

Narrated By Jabir bin 'Abdullah : 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.