Abu Dawud
General Behavior
Book Number: 036 / Hadith Number: 5069
Narated By Uthman ibn Affan : Aban ibn Uthman said: I heard Uthman ibn Affan (his father) say: I heard the Apostle of Allah (pbuh) say: If anyone says three times: "In the name of Allah, when Whose name is mentioned nothing on Earth or in Heaven can cause harm, and He is the Hearer, the Knower" he will not suffer sudden affliction till the morning, and if anyone says this in the morning, he will not suffer sudden affliction till the evening.
Aban was afflicted by some paralysis and when a man who heard the tradition began to look at him, he said to him: Why are you looking at me? I swear by Allah, I did not tell a lie about Uthman, nor did Uthman tell a lie about the Prophet (pbuh), but that day when I was afflicted by it, I became angry and forgot to say them.