Explaining the tradition “Every child is a born on Islam”, Hammad bin 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.
Hadith Referenceسنن ابي داود / كتاب السنة / 4716
Hadith Gradingالألبانی:صحيح الإسناد مقطوع | زبیر علی زئی:إسناده صحيح
1؎: The interpretation of being born upon the fitrah of Islam is explained in this way: since, according to the divine covenant (mithaq ilahi), everyone affirmed the oneness of Allah, therefore, each person is born upon that affirmation. Later, people make him a Jew, Christian, Magian, or polytheist.