The Prophet Muhammad is the final messenger and it is inconceivable knowing the mercy of the merciful that He would leave humans without any form of clear guidance. In other words, what He gave this final message must be suitable to guide mankind after him. In fact, the Prophet himself made this very same point in essence when he said, “I have left with you two things that if you cling to them you will never be misguided after me: The Book of Allah and the Sunnah of His Messenger.”
In addition to the fact that the Prophet Muhammad (peace and blessings of Allah be upon him) is the final prophet, Allah has also said, “This day, I have perfected your religion for you, completed My Favor upon you, and have chosen for you Islam as your religion” (5:3). Allah has also declared that the Prophet Muhammad (peace and blessings of Allah be upon him) is the prophet for all of mankind: “Say [O Muhammad], ‘O Mankind, indeed I am the Messenger of Allah to you all” (7:158).