Al-Adab Al-Mufrad
Book 55, Hadith 5
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.
Unreviewed translation. All hadiths in this collection are available in five languages. Titles and introductions may remain in Arabic.
And from ‘Uqbah ibn Khalid al-Sakuni, who said: Abu Sa‘d Sa‘id ibn Marzuban related to us: I heard Anas ibn Malik say: The Messenger of Allah, Allah bless him and grant him peace, said: ‘People will keep asking about what does not exist until they say: “Allah is the Creator of everything, so who created Allah?”’
Notes and open questions — unreviewed
The Arabic gives the question as the endpoint of people’s asking; the preceding clause says Allah is the Creator of everything.