Jami` at-Tirmidhi: Book 48, Hadith 38

A man from Banu Hanzalah said: “I accompanied Shaddad bin Aws on a journey, so he said: ‘Should I not teach you what the Messenger of Allah used to teach us? That you say: “O Allah, I ask You for steadfastness in the affair and I ask You for determination upon guidance, and I ask You to make me grateful for Your favor, and excellence in worshiping You, and I ask You for a truthful tongue and a sound heart, and I seek refuge in You from the evil of what You know, and I ask You for the good of what You know, and I seek Your forgiveness for that which You know. Verily, You are the Knower of all that is hidden (Allāhumma innī as’alukath-thabāta fil-amri, wa as’aluka `azīmatar-rushdi, wa as’aluka shukra ni`matika, wa ḥusna `ibādatika, wa as’aluka lisānan ṣādiqan wa qalban salīman, wa a`ūdhu bika min sharri mā ta`lamu, wa as’aluka min khairi mā ta`lamu, wa astaghfiruka mimmā ta`lamu innaka anta `allāmul-ghuyūb).’” He said: ‘The Messenger of Allah (ﷺ) said: “There is no Muslim who lays down to sleep while reciting a Surat from Allah’s Book, except that Allah entrusts an angel, so that nothing approaches him to harm him until he awakens, whenever he awakens.”