Abu Hurairah (RA) narrates: Rasulullah (SAW) came to us and said: Take protection with your shields! Sahabah (Prophet's companions) asked: O Rasulullah! is there a threat of some enemy coming? He said: Take protection with your shields against the Fire (of Hell)! Say, Subhanallah (Allah is flawless), Alhamdulillah (Praise be to Allah), La ilaha illallah (None is worthy of worship but Allah), Allahu Akbar (Allah is the Greatest). On the Day of Resurrection, these words will come from the front, back, right and left (defence from all sides) of the one who had recited them, and ensure deliverence for him, and these are those good deeds for which the reward will be given eternally. Source: Tabarani.