Sahih Al Bukhari (512)

أخبرنا جابر بن عبدالله {أن النبي صلى الله عليه وسلم جاءه عمر بن الخطاب يوم الخندق فقال يا رسول الله والله ما كدت أن أصلي حتى كادت الشمس تغرب وذلك بعد ما أفطر الصائم فقال النبي صلى الله عليه وسلم والله ما صليتها فنزل النبي صلى الله عليه وسلم إلى بطحان وأنا معه فتوضأ ثم صلى يعني العصر بعد ما غربت الشمس ثم صلى بعدها المغرب}

Jabir ibn `Abdullah (may Allah be pleased with him) narrated: On the Day of Al-Khandaq (the trench), `Umar ibn Al-Khattab went to the Prophet (peace be upon him) and said, “O Allah’s Messenger! By Allah, I could not pray (the `Asr) till the sun had set.” `Umar told this to the Prophet (peace be upon him) at the time when a fasting person had done Iftar (taken his meals). The Prophet (peace be upon him) then went to Buthan and I was with him. He performed ablution and offered the `Asr Salah after the sun had set and then the Maghrib Salah.