(Then you should desist from doing what you are in doubt about.)
Al-Bukhari, cf. Al-Asqalani, Fathul-Bari 6/336, Muslim 1/120.
Hisnul Muslim 133
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For those unfamiliar, Hisnul Muslim (Fortress of the Muslim) is a well-known du'a book compiled by Said Bin Ali Bin Wahf Al-Qahtani. It categorizes authentic supplications that the Prophet ﷺ made in various situations. In the 40th chapter, "For one afflicted with doubt in his faith" there are 3 supplications that you can learn and recite.
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We've created a video for each du'a to try and make it easy to learn. There's also transliteration and translation for every du'a. Lastly, after pressing the play button you can tap on any word and it'll skip to that part in the audio.