Hadith: O Messenger of Allah, I could not offer the ‘Asr prayer till the sun had set. The Prophet (may Allah's peace and blessings be upon him) said: By Allah! I have not yet prayed it either. So we went to But-hān, and he performed ablution and so did we, and then he offered the ‘Asr prayer after the sun had set and then followed it with the Maghrib prayer

عن جابر بن عبد الله رضي الله عنهما «أنَّ عمر بن الخَطَاب رضي الله عنه جاء يَومَ الخَندَقِ بَعدَ مَا غَرَبَت الشَّمسُ فَجَعَل يَسُبُّ كُفَّار قُرَيشٍ، وقال: يا رسول الله، مَا كِدتُّ أُصَلِّي العَصرَ حَتَّى كَادَت الشَّمسُ تَغرُبُ، فَقَال النَبِيُّ صلى الله عليه وسلم : والله مَا صَلَّيتُهَا، قال: فَقُمنَا إلَى بُطحَان، فَتَوَضَّأ للصَّلاَة، وتَوَضَأنَا لَهَا، فَصَلَّى العَصر بعد مَا غَرَبَت الشَّمسُ، ثُمَّ صَلَّى بعدَها المَغرِب».
[صحيح] - [متفق عليه]

Jābir ibn ‘Abdullāh (may Allah be pleased with him) reported: ‘Umar ibn al-Khattāb came cursing the disbelievers of Quraysh after the sun had set on the Day of the Trench, and he said: "O Messenger of Allah, I could not offer the ‘Asr prayer till the sun had set." The Prophet (may Allah's peace and blessings be upon him) said: "By Allah! I have not yet prayed it either." So we went to But-hān, and he performed ablution and so did we, and then he offered the ‘Asr prayer after the sun had set and then followed it with the Maghrib prayer.
[Authentic hadith] - [Narrated by Bukhari & Muslim]

Explanation

‘Umar ibn al-Khattāb (may Allah be pleased with him) came to the Prophet (may Allah's peace and blessings be upon him) on the Day of the battle of the Trench after the sun had set, and he was cursing the disbelievers of Quraysh, because they had distracted him from offering the ‘Asr prayer, and he could not pray it until after the sun had set. So the Prophet (may Allah's peace and blessings be upon him) swore by Allah, and he is the truthful, that he too had not yet prayed it, in order to calm ‘Umar (may Allah be pleased with him) because it was hard for him. Then the Prophet (may Allah's peace and blessings be upon him) got up and performed ablution, and the Companions did so too, and then he offered the ‘Asr prayer after the sun had set, followed by the Maghrib prayer.