Whoever says it sincerely from the heart, Allah shall protect him from the hellfire!

Attban ibn Malik said: The Prophet (Peace Be upon Him) said: "Allah has made the fire of Hell unlawful for him who says La ilaha illallah (none has the right to be worshipped except Allah),' seeking His Pleasure." Sahih al-Bukhari (425), Sahih Muslim (1/455).

Omar ibn al-Khattab, may Allah be pleased with him, said: I heard Allah's Messenger, Peace Be upon Him, saying: "I know a word, if any servant says it sincerely from the heart and die believing in it, Allah will make the hellfire unlawful to him, 'La ilaha illallah'"

(Al-Mustadarak ala al-Sahehayn, by al-Hakim) (1/143).