Allah (S) has given every single Prophet, one request – one supplication that is guaranteed to be accepted. The Prophets of the past have used this to ask something that they really wanted to see fulfilled. Some prophets used this against their own people because they used to get so frustrated. For example, Nuh (A) after preaching for nine hundred and fifty years, got frustrated at his people for still not listening to his message and made this supplication,
O Allah I don’t want you to leave one house of disbelievers on this earth.
Thereafter, Allah destroyed his city, which was all of humanity at that time.
Similarly Ibrahim (A) made a supplication to Allah (S) for Prophet Muhammad , asking for the messenger to come from his progeny. That is why Prophet Muhamamd later said, I’m the Dua of my father Ibrahim. Musa (A) made supplication against Feraun, O Allah do not guide this person. That is exactly what happened, Feraun died as a disbliever. Suleiman (A) made supplication to Allah – give me a dominion that you have given to no other human before me. So Allah (S) gave him power that no one had before him.
The Prophet had the greatest of the supplications and what did he use it for? The Prophet said,
Allah has given one Dua to every single prophet and every single messenger, that he has guaranteed that he will respond to. And every single prophet has used up this Dua for himself in this world, except for me. I have saved it and I have not used it and I will not use it in this life. I have kept it for my Ummah and I will use it for them on the day of judgement. And my Dua will be, O Allah, forgive my entire Ummah.
This is the Dua of the Prophet , that anybody who believed in me, O Allah forgive him. Allah will accept this Dua and every single Muslim will eventually enter Jannah. Every single person who believed in him and acted upon it even a little bit, no matter how sinful that person is, will eventually enter Paradise.
So here we see, how the Prophet saved this one Dua for us, that he could have used for himself. But he chose to make the sacrifice for his Ummah, this shows us the love that he had for all of us.