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 } .
AbuDaud
Model Behavior of the Prophet (Kitab Al-Sunnah) • Hadith 4716