Saad was wounded at the Battle of the Trench. A man from Quraysh named Hibban bin al-Ariqah shot him with an arrow that struck the median vein of his arm. The Prophet ﷺ pitched a tent for him in the mosque so that he could visit him from nearby.
أصيب سعد الخندق، رماه رجل يوم من قريش يقال له: حبان بن العرقة، رماه في الأكحل، فضرب النبي ﷺ خيمة في المسجد ليعوده من قريب.
Bukhari reported it in the Book of Military Expeditions (4122), and Muslim in the Book of Jihad (1769), both from the hadith of Abdullah bin Numayr, who said, Hisham narrated to us from his father, from Ayesha, and he mentioned it. Al-Akhal is the vein in the middle of the forearm.