Tafsir of Al-Fath 48:23

(The established way of Allah which has occurred before): It is in the accusative case as a verbal noun (masdar) derived from an omitted verb; meaning: "Allah has established the practice of granting victory to His prophets, peace be upon them, as an ancient practice regarding those who have passed among the nations," just as He, Glorified be He, said: "I will surely overcome, I and My messengers." This is according to the meaning that is immediately understood from it. Perhaps what is meant is that His practice—Exalted be He—is that the ultimate outcome belongs to His prophets, peace be upon them, not that whenever they fought the disbelievers, they overcame and defeated them. (And you will not find in the way of Allah any change): or any alteration.