Sunan Abi Dawud, Hadith 4716

Explaining the tradition “Every child is a born on Islam”, Hammad b. Salamah said : In our opinion it means that covenant which Allah had taken in the loins of their fathers when He said : “Am I not your Lord? They said: Yes.

حَدَّثَنَا الْحَسَنُ بْنُ عَلِيٍّ، حَدَّثَنَا حَجَّاجُ بْنُ الْمِنْهَالِ، قَالَ سَمِعْتُ حَمَّادَ بْنَ سَلَمَةَ، يُفَسِّرُ حَدِيثَ ‏"‏ كُلُّ مَوْلُودٍ يُولَدُ عَلَى الْفِطْرَةِ ‏"‏ ‏.‏ قَالَ هَذَا عِنْدَنَا حَيْثُ أَخَذَ اللَّهُ عَلَيْهِمُ الْعَهْدَ فِي أَصْلاَبِ آبَائِهِمْ حَيْثُ قَالَ ‏{‏ أَلَسْتُ بِرَبِّكُمْ قَالُوا بَلَى ‏}‏ ‏.‏

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also referenced under: Hadith 4698 (Hasan)