Shama-il Tirmidhi Chapter 34, Hadith Number 9 (224).
Aamir bin Saâ€™d radiyallahu anhu says, My father Saâ€™d said, â€˜Rasoolullah sallallahu alaihe wasallam laughed on the day of the Battle of Khandaq till his teeth showed.â€™ Aamir radiyallahu anhu says, â€˜I asked why did he laugh?â€™ he replied, â€˜A Kaafir had a shield, and Saâ€™d was a great archer. The kaafir protected himself by swaying the shield from side to side covering his forehead. (Sayyidina Saâ€™d radiyallahu anhu was a famous marksman, but the kaafir did not let the arrows get him). Saâ€™d radiyallahu anhu took an arrow (and kept it ready in the bow). When the non believer removed the shield from his head, he quickly aimed at the kaafir and did not miss the target, i.e. the (enemyâ€™s) forehead. The enemy immediately fell down, his legs rising into the air. On that Rasoolullah sallallahu alaihe wasallam laughed till his mabaraak teeth showed.â€™ I asked, â€˜Why did Rasoolullah sallallahu alaihe wasallam laugh?â€™ He replied, â€˜Because of what Saâ€™ad had done to the man.â€™
It may be doubted that Sayyidina Rasoolullah sallallahu alaihe wasallam laughed because the manâ€™s feet were raised in the air, hence the satr (private parts) was exposed. Therefore the reason for laughing had to be asked. Sayyidina Saâ€™d radiyallahu anhu replied, No, but because I, although a good marksman, the non believer protected himself. He was clever to move his shield from side to side. But Saâ€™d thought of a plan, and aimed at him in such a way that he did not have a chance to protect himself and fell down.