Sahih Bukhari Volume 004, Book 053, Hadith Number 408.

Narated By Al-Bara : When the Prophet intended to perform the 'Umra he sent a person to the people of Mecca asking their permission to enter Mecca. They stipulated that he would not stay for more than three days and would not enter it except with sheathed arms and would not preach (Islam) to any of them. So Ali bin Abi-Talib started writing the treaty between them. He wrote, "This is what Muhammad, Apostle of Allah has agreed to." The (Meccans) said, "If we knew that you (Muhammad) are the Apostle of Allah, then we would not have prevented you and would have followed you. But write, 'This is what Muhammad bin 'Abdullah has agreed to...' " On that Allah's Apostle said, "By Allah, I am Muhammad bin 'Abdullah, and, by Allah, I am Apostle of 'Allah." Allah's Apostle used not to write; so he asked 'Ali to erase the expression of Apostle of Allah. On that 'Ali said, "By Allah I will never erase it." Allah's Apostle said (to 'Ali), "Let me see the paper." When 'Ali showed him the paper, the Prophet erased the expression with his own hand. When Allah's Apostle had entered Mecca and three days had elapsed, the Meccans came to 'Ali and said, "Let your friend (i.e. the Prophet) quit Mecca." Ali informed Allah's Apostle about it and Allah's Apostle said, "Yes," and then he departed.