He should seek refuge in Allah
He should renounce that which is causing such doubt.
He should say:
آمَنْـتُ بِاللهِ وَرُسُـلِه.
Amantu billahi warusulih.
‘I have believed in Allah and His Messenger.’
He should also recite the following verse:
هُوَ الأوَّلُ، وَالآخِـرُ، وَالظّـاهِـرُ، وَالْبـاطِـنُ، وَهُوَ بِكُلِّ شَيءٍ عَلـيم.
Huwa al-awwalu, waal-akhiru, waalththahiru waalbatinu wahuwa bikulli shayin AAaleem.
‘He is The First and The Last, Aththahir and Al-Batin and He knows well all things.’
Aththahir: 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.