The Messenger of Allah ﷺ said, "Whoever is thrown from his mount in the cause of Allah is a martyr."
قال رسول الله ﷺ: من صرع عن دابته في سبيل الله فهو شهيد.
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Ibn Abu Asim reported it in Jihad (237), and Tabarani in Al-Kabir (17/323), both from the hadith of Abdullah bin Wahb, from Amr bin Harith, from Ubaydullah bin Abu Jafar, from Jafar bin Abdullah, from Uqba bin Amir, and he related it. Its isnad is sahih. Haythami said in Al-Majmu (5/301), "Tabarani reported it, and its narrators are trustworthy." The hadith has other routes, the most sahih of which is the one I have mentioned.