Muslims call him a prophet. My question is what is the prophecy of Muhammad? What did he predict?
Two words are used in the Qur’an to describe what translators call a ‘Messenger’ or a ‘Prophet’. These words are: Nabī (‘Prophet’) and Rasūl (‘Messenger’).
In the Islamic context, a Nabī is a man sent by Allāh (subḥānahu ūta’āla) to provide guidance, or give warning, to those who are already under the Law; whereas a Rasūl (‘Messenger’) is charged with delivering the Law in the first place – in the form of scripture (he is also required to provide guidance and give warning, of course). All Rasūl are considered to be Nabī; but not all Nabī were Rasūl.
Muhammad (sallallahu 'alayhi wa sallam) was a Rasūl.
Prophets are not fortunetellers. Their statements are conditional; and their purpose is not to predict the future but to bring about a change of mind – a change of behavior – on the part of their listeners. For example:
‘Then the word of Yahweh came to me as follows: “House of Israel, can I not do to you what this potter does? Yahweh demands. Yes, like clay in the potter’s hand, so you are in mine, House of Israel. Sometimes I announce that I shall uproot, break down and destroy a certain nation or kingdom, but should the nation I have threatened abandon its wickedness, I then change my mind about the disaster which I had intended to inflict on it. Sometimes I announce that I shall build up and plant a certain nation or kingdom, but should that nation do what displeases me and refuse to listen to my voice, I then change my mind about the good which I was intending to confer on it. So now, say this to the people of Judah and the inhabitants of Jerusalem, “Yahweh says this: Listen, I am preparing a disaster for you, I am working out a plan against you. So now, each one of you, turn back from your evil ways, amend your conduct and actions.”’ (Jeremiah: 5-11).
As you can see, the Exalted may not send the predicted punishment if the people repent; or He may withhold a predicted blessing if they do evil in His sight.
Compare:
‘(This is) a Scripture whose verses are perfected, then set out clearly, from One who is all wise, all aware. (Say, Prophet), “Worship no one but Allāh. I am sent to you from Him to warn and to give good news. Ask your Lord for forgiveness, then turn back to Him. He will grant you wholesome enjoyment until an appointed time, and give His grace to everyone who has merit. But if you turn away, I fear you will have torment on a terrible Day: it is to Allāh that you will all return, and He has power over everything.”’ (Hud: 1-4).