For one in doubt about their faith

يَسْتَعِيذُ بِاللَّهِ, يَنْتَهِي عَمَّا وَسْوَسَ فِيهِ

(When the devil whispers doubt) one should seek refuge in Allah (saying: "I seek refuge in Allah from the accursed devil") and cease that about which the doubt is being whispered.

Source: Al-Bukhari, Muslim 1/120.

يَقُولُ: آمَنْتُ بِاللَّهِ وَرُسُلِهِ

(He should say: (I have believed in Allah and His Messenger.))

Source: Muslim, 1/119-20.

يَقْرَأُ قَوْلَهُ تَعَالَى: ﴿هُوَ الْأوَّلُ وَالْآخِرُ وَالظّاهِرُ وَالْباطِنُ وَهُوَ بِكُلِّ شَيْءٍ عَلِيمٌ﴾

(He should also recite the following verse: (He is The First and The Last, Aththahir and Al-Batin and He knows well all things.)(ththahir: Indicates the greatness of His attributes and the insignificance of every single creation in respect to His greatness and Highness, for He is above all of His creation as regards His essence and attributes.)(Al-Batin: Indicates His awareness and knowledge of all secrets, of that which is in the hearts and the most intimate of things just as it indicates His closeness and nearness to all in a manner which befits His majesty.))

Source: Al-Hadid 57:3, Abu Dawud 4/329.