Sahih Bukhari, Chapter: Jihaad (Fighting for the cause of Allah)

My uncle Anas bin An-Nadr was absent from the Battle of Badr. He said, "O Allah's Apostle (SAW)! I was absent from the first battle you fought against the Mushrikun (Pagans). (By Allah) if Allah gives me a chance to fight the Mushrikun (pagans), no doubt. Allah will see how (bravely) I will fight." On the day of Uhud when the Muslims turned their backs and fled, he said, "O Allah! I apologize to You for what these (i.e. his companions) have done, and I denounce what these Mushrikun (i.e. the pagans) have done." Then he advanced and Sad bin Muadh met him. He said "O Sad bin Muadh! By the Lord of An-Nadr, Paradise! I am smelling its aroma coming from before (the mountain of) Uhud," Later on Sad said, "O Allah's Apostle (SAW)! I cannot achieve or do what he (i.e. Anas bin An-Nadr) did. We found more than eighty wounds by swords and arrows on his body. We found him dead and his body was mutilated so badly that none except his sister could recognize him by his fingers." We used to think that the following Verse was revealed concerning him and other men of his sort: "Among the believers are men who have been true to their covenant with Allah..." till the end if the verse (33.23) His sister Ar-Rubai broke a front tooth of a woman and Allah's Apostle (SAW) ordered for retaliation. On that Anas (bin An-Nadr) said, "O Allah's Apostle (SAW)! By Him Who has sent you with the Truth, my sister's tooth shall not be broken." Then the opponents of Anas's sister accepted the compensation and gave up the claim of retaliation. So Allah's Apostle (SAW) said, "There are some people amongst Allah's slaves whose oaths are fulfilled by Allah when they take them."