Sahl bin Hunayf (may Allah be pleased with him) died in Kufah

Muhammad bin Umar states that Sayyiduna Sahl bin Hunayf (may Allah be pleased with him) took part in the Battles of Badr and Uhud, and on the day of Uhud, when the people had scattered, he stood firm with the Messenger of Allah (peace and blessings be upon him). He pledged allegiance to him (peace and blessings be upon him) unto death, and that day he kept shooting arrows in defence of him (peace and blessings be upon him). Then the Messenger of Allah (peace and blessings be upon him) said: "Supply Sahl with arrows, for he is sahl (that is, generous and gentle)." He also took part in the Battle of al-Khandaq and in all the battles alongside the Messenger of Allah (peace and blessings be upon him), and he also took part in the Battle of Siffin with Sayyiduna Ali bin Abi Talib (may Allah be pleased with him).

Muhammad bin Abi Umamah bin Sahl narrates from his father that Sayyiduna Sahl bin Hunayf (may Allah be pleased with him) died in Kufa after the return from the Battle of Siffin, in the year 38 AH, and the Commander of the Faithful, Ali bin Abi Talib (may Allah be pleased with him), led his funeral prayer.

It is narrated from Sayyiduna Abdullah bin Ma'qil (may Allah be pleased with him) that Sayyiduna Ali (may Allah be pleased with him) led the funeral prayer of Sayyiduna Sahl bin Hunayf (may Allah be pleased with him) and said six takbirs in it. Then he turned to us and said: "Indeed, he was one of the people of Badr."

Abu Umamah bin Sahl narrates from his father (Sayyiduna Sahl bin Hunayf (may Allah be pleased with him)) that he said: O my dear son! I saw us on the day of Badr, when one of us would point his sword towards the head of an idolater, and his head would fall from his body before the sword reached him.
The chain of this hadith is sahih (authentic), but the two Shaykhs (Bukhari and Muslim) did not record it.