Sayyiduna Imran bin Husayn (may Allah be pleased with him) said: I heard the Messenger of Allah (peace and blessings be upon him) say: "Whoever knows with certainty that Allah is his Lord, and that I am his Prophet, truthfully from the depths of his heart" (and he (peace and blessings be upon him) pointed with his hand to the skin of his chest) "Allah makes his flesh unlawful for the Fire."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 15344