Al-Adab Al-Mufrad, Book 55, Hadith 1286 (English) ⓘ

Anas ibn Malik said, "The Messenger of Allah, may Allah bless him and grant him peace, said, 'People will continue to ask about things that do not exist to such an extent that they will say, "Allah created everything, so who created Allah?"'"

وَعَنْ عُقْبَةَ بْنِ خَالِدٍ السَّكُونِيِّ، قَالَ‏:‏ حَدَّثَنَا أَبُو سَعْدٍ سَعِيدُ بْنُ مَرْزُبَانَ قَالَ‏:‏ سَمِعْتُ أَنَسَ بْنَ مَالِكٍ يَقُولُ‏:‏ قَالَ رَسُولُ اللهِ صلى الله عليه وسلم‏:‏ لَنْ يَبْرَحَ النَّاسُ يَسْأَلُونَ عَمَّا لَمْ يَكُنْ، حَتَّى يَقُولُوا‏:‏ اللَّهُ خَالِقُ كُلِّ شَيْءٍ، فَمَنْ خَلَقَ اللَّهَ‏.‏