Sahih Muslim 133
Sahih Muslim · Book 1: The Book of Faith · Clarifying the Waswasah (Whispers, Bad Thoughts) with regard to faith, and what the one who experiences that should say
It is narrated on the authority of 'Abdullah b. Mas'ud that the Apostle (ﷺ) was asked about evil prompting, to which he replied:
حَدَّثَنَا يُوسُفُ بْنُ يَعْقُوبَ الصَّفَّارُ ، حَدَّثَنِي عَلِيُّ بْنُ عَثَّامٍ ، عَنْ سُعَيْرِ بْنِ الْخِمْسِ ، عَنْ مُغِيرَةَ ، عَنْ إِبْرَاهِيمَ ، عَنْ عَلْقَمَةَ ، عَنْ عَبْدِ اللَّهِ ، قَالَ سُئِلَ النَّبِيُّ صلى الله عليه وسلم عَنِ الْوَسْوَسَةِ قَالَ " تِلْكَ مَحْضُ الإِيمَانِ " .
It is pure faith.