Imam al-Shafi'i (may Allah have mercy on him) said: A man of the Ansar stayed behind the people of Bi'r Ma'unah. He saw birds settled over the bodies of his companions, so he said to Amr bin Umayyah (may Allah be pleased with him): "I too will fight the enemy until they kill me, and I will not stay back from the place where my companions were killed." He did so and was martyred. Amr bin Umayyah (may Allah be pleased with him) returned and informed the Prophet (peace and blessings be upon him), and he (peace and blessings be upon him) said good words about him. And it is said that he (peace and blessings be upon him) said to Amr (may Allah be pleased with him): "Why did you not advance and fight until you were killed?" Imam al-Shafi'i (may Allah have mercy on him) said: The Messenger of Allah (peace and blessings be upon him) sent Amr bin Umayyah al-Damri (may Allah be pleased with him) and a man of the Ansar as a detachment by themselves, and sent Abdullah bin Unays (may Allah be pleased with him) alone as a detachment.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 18200