Aftar he declared his messege, even his enemies acknowledged his unblemished character. Although "Abu Sufian" was the prophet's bitter enemy, he couldn't help saying to Heraculius, the Byzantine emperor the prophet Muhammed had never told a lie.
Another piece of evidence that Muhammed was a true prophet is that the prophet and the early muslims sufferd great hardships from the cruelty of pagans of Mecca. They at first were standerd and mocked, then , the pagan attacked individual Muslims trying to humiliate them. Then the pagans resorted to complete boycott where the Muslims were besieged in their quarter at Mecca.
During the boycott no one was allowed to deal with them, no buying, no selling , no social relations of any kind. The prophet endured all hardships with a firm belief in the final victory he was promised by Allah.
During the boycott his uncle and his wife, who were the greatest protectors, died and became defenseless but he didn't weaken or yield. He visited a tribe called "Thakif" in "Al-Taef" to inform them of his messege, but they greeted him with little sympathy . Their leader mocked him and their children and riffraff threw him with stons.
During the battle of " Hunain" the Muslims faced a difficult situation despite their great number. Muslims were about to suffer defeat , In his critical situation propet Muhammed rushed forward until he faced the enemy and shouted "Allahu Akbar" I am the prophet , no lie, I am the son of Abdul Muttalib". These words aroused the Muslims an emerged victory.
There is a states man who was converted to Islam is called Sir Jalaluddin lauder. He said that it was impossible to doubt the earnestness of the prophet message.
The chiefs of Mecca went to Abu Taleb and asked him to make Muhammed dissuade from his course and they would make him the richest one. If he wanted honour and fame they were ready to crown him as their King. If he refused , they wanted from Abu-Taleb to withdraw his protection and allow them to deal freely with him.
The prophet refused and said : "By Allah, O uncle! Even they put the sun in my right hand and the moon in my left hand, i will never renounce this claim. I'll fight for it either until it is fully acknowledged or until I die.
Abu-Taleb said to him : "Proceed in your course, O nephew, I shall never forsake you."