Sahih Bukhari Volume 5, Book 59, Hadith Number 582.

Narrated By Anas bin Malik : On the day of the Conquest, the Prophet entered Mecca, wearing a helmet on his head. When he took it off, a man came and said, “Ibn Khatal is clinging to the curtain of the Ka’ba.” The Prophet said, “Kill him.” (Malik a sub-narrator said, “On that day the Prophet was not in a state of Ihram as it appeared to us, and Allah knows better.”)