Quran 33:40 states that Muhammad is not the father of any man among the believers but is the messenger of God and the seal of the prophets, khatam al-nabiyyin, and that God has knowledge of all things. Classical commentators from the earliest tafsir onward read khatam as a literal seal that closes something, understanding the verse to mean that the line of prophecy running through Adam, Noah, Abraham, Moses and Jesus ends with Muhammad, and that no prophet, reformer or law bringer of any rank can follow him.
Hadith collections reinforce the same reading. Sahih al-Bukhari and Sahih Muslim both preserve a saying in which Muhammad compares himself to the final brick that completes a wall of prophets, a building admired by everyone except for the one missing stone, which he says he supplies. Another widely transmitted hadith has Muhammad warn that around thirty false claimants to prophecy will appear before the Day of Judgment, framing any future claim to the office as an anticipated form of deception rather than a live theological possibility. On this reading, held without significant internal dissent by Sunni and Shia scholarship across the classical period, finality of prophethood functions as a boundary marker for the religion itself: whatever a movement teaches, if it involves a new claimant to the prophetic office, it has stepped outside Islam by that fact alone, regardless of anything else it holds in common with it.