كتاب الإيمان
1
The Book of Faith
(60)
Chapter: Clarifying the Waswasah (Whispers, Bad Thoughts) with regard to faith, and what the one who experiences that should say
(60)
باب بَيَانِ الْوَسْوَسَةِ فِي الإِيمَانِ وَمَا يَقُولُهُ مَنْ وَجَدَهَا ‏‏

Anas b. Malik transmitted it from the Messenger of Allah (ﷺ) that the Great and Glorious Allah said:

Verily your people would constantly question about this and that till they would say: Well, it is Allah Who created the creation, but who created Allah?
حَدَّثَنَا عَبْدُ اللَّهِ بْنُ عَامِرِ بْنِ زُرَارَةَ الْحَضْرَمِيُّ، حَدَّثَنَا مُحَمَّدُ بْنُ فُضَيْلٍ، عَنْ مُخْتَارِ بْنِ فُلْفُلٍ، عَنْ أَنَسِ بْنِ مَالِكٍ، عَنْ رَسُولِ اللَّهِ صلى الله عليه وسلم قَالَ ‏ "‏ قَالَ اللَّهُ عَزَّ وَجَلَّ إِنَّ أُمَّتَكَ لاَ يَزَالُونَ يَقُولُونَ مَا كَذَا مَا كَذَا حَتَّى يَقُولُوا هَذَا اللَّهُ خَلَقَ الْخَلْقَ فَمَنْ خَلَقَ اللَّهَ ‏"‏ ‏.‏
Reference : Sahih Muslim 136 a
In-book reference : Book 1, Hadith 258
USC-MSA web (English) reference : Book 1, Hadith 250
  (deprecated numbering scheme)