He should seek refuge in Allaah.

أَعُوذُ بِاللَّهِ مِنَ الشَّيْطَانِ الرَّجِيمِ

I seek refuge in Allah from Satan the outcast.

'A'oothu billaahi minash-Shaytaanir-rajeem.

He should renounce that which is causing such doubt [1] He should say:

آمَنْتُ بِاللَّهِ وَرُسُلِهِ

I believe in Allah and His Messenger. [2]

'Aamantu billaahi wa Rusulihi.

[1] Al Bukhaaree with al-Fath 6/336 and Muslim 1/120 [2] Muslim 1/119,120.