It is narrated from Sayyiduna Salman (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Whoever says: 'O Allah! I call You to witness, and Your angels and the bearers of Your Throne, and I call to witness whatever is in the heavens and the earth, that You are Allah, there is no true god except You, You alone, You have no partner; and I bear witness that Muhammad (peace and blessings be upon him) is Your servant and Your Messenger.' Whoever says this once, Allah will free a third of him from the Fire; whoever says it twice, Allah will free two thirds of him from the Fire; and whoever says it three times, Allah will free him entirely from the Fire."
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 6037