Sunan Abi Dawud, Hadith 4703

Abu Hurairah reported to the Messenger of Allah( may peace be upon him) as sayings: People will continue to ask one another(questions) till this is pronounced: Allah created all things, but who created Allah ? Whoever comes across anything of that, he should say: I believe in Allah.

حَدَّثَنَا هَارُونُ بْنُ مَعْرُوفٍ، حَدَّثَنَا سُفْيَانُ، عَنْ هِشَامٍ، عَنْ أَبِيهِ، عَنْ أَبِي هُرَيْرَةَ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏ "‏ لاَ يَزَالُ النَّاسُ يَتَسَاءَلُونَ حَتَّى يُقَالَ هَذَا خَلَقَ اللَّهُ الْخَلْقَ فَمَنْ خَلَقَ اللَّهَ فَمَنْ وَجَدَ مِنْ ذَلِكَ شَيْئًا فَلْيَقُلْ آمَنْتُ بِاللَّهِ ‏" ‏ ‏.‏

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also referenced under: Hadith 4721 (DarusSalam)