Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: The Messenger of Allah (peace and blessings be upon him) said on the day of the Conquest of Makkah: "Indeed, this city is sacred; Allah has made it sacred. Killing in it was not made lawful for anyone before me, and it was made lawful for me; so it is sacred by the sanctity of Allah."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10897