It is narrated from Sayyiduna Ali (may Allah be pleased with him) that he said: The Prophet (peace and blessings be upon him) set out for Khaybar. When he reached it, he (peace and blessings be upon him) sent Sayyiduna Umar (may Allah be pleased with him), and sent people with him to the enemy's town or fortress. They fought them, but before long (the enemy) forced Umar and his companions to turn back. When they came back, they were calling one another cowards, and this situation displeased the Prophet (peace and blessings be upon him). This hadith has an authentic (sahih) chain of narration, but the two Shaykhs (Bukhari and Muslim) did not record it.