Abu Musa Al-Hanafi reported that Imam Ali (a.) said:“When the Messenger of God (s.) advanced to Khaibar and arrived there, he sent Umar and the people with him to their city or castle and they fought against them. After a while they defeated Umar and his companions so that they returned, while they called him a coward and he accused them of cowardice. After this, the messenger of God (s.) departed.” Al-Hakim An-Naisaburi writes: “This tradition is authentic (Sahih) according to its chain, but the two (Al-Bukhari and Muslim) did not publish it.” Muhammad Ibn Ahmad Adh-Dhahabi writes: “Authentic (Sahih).” [Al-Mustadrak, volume 3, page 40, Hadith 4340]

أخبرنا أبو العباس محمد بن أحمد المحبوبي بمرو ثنا سعيد بن مسعود ثنا عبيد الله بن موسى ثنا نعيم بن حكيم عن أبي موسى الحنفي عن علي عليه السلام: قال: سار النبي صلى الله عليه و سلم إلى خيبر فلما أتاها بعث عمر وبعث معه الناس إلى مدينتهم أوقصرهم فقاتلوهم فلم يلبثوا أن هزموا عمر أصحابه فجاءوا يجبنونه ويجبنهم فسار النبي صلى الله عليه وسلم وقال الحاكم النيسابوري: هذا حديث صحيح الإسناد و لم يخرجاه وقال محمد بن أحمد الذهبي: صحيح

Ibn Jarir reported that Kulaib said: “Umar gave a sermon on Friday and read the Surah Al Imran and he liked it. As he preached and recited, he came to the point: “Those of you who turned away.” (3:155) Umar said: “When the day of Uhud came, we gave them a defeat, but I ran away until I climbed a hill and I saw myself climbing up as if I were a mountain goat.” The people said: “Muhammad was killed!” I said: “I will not find anyone to say that Muhammad has been killed except I kill him.” Until we got together on the hill. At this point the verse was sent down: “Those of you who turned away on the day when the two hosts met, Satan has made them stumble because of some of what they have acquired.” [Ad-Durr-ul-Manthur of As-Suyuti, Volume 4 page 81]

أخرج ابن جرير عن كليب قال خطب عمر يوم الجمعة فقرأ آل عمران وكان يعجبه إذا خطب ان يقرأها فلما انتهى إلى قوله ان الذين تولوا منكم يوم التقى الجمعان قال لما كان يوم أحد هزمناهم ففررت حتى صعدت الجبل فلقد رأيتني أنزو كأنني أروى والناس يقولون قتل محمد فقلت لا أجد أحدا يقول قتل محمد الا قتلته حتى اجتمعنا على الجبل فنزلت ان الذين تولوا منكم يوم التقى الجمعان الآية كلها

Comment:

Abu Musa Al-Hanafi is trustworthy [Here!]