Hajjaj bin Minhal (may Allah have mercy on him) said: I heard Hammad bin Salamah (may Allah have mercy on him) explaining the hadith "Every child is born upon the fitrah (natural disposition)." He said: In our view this refers to when Allah Almighty took the covenant from them in the loins of their fathers, when Allah Almighty said: "Am I not your Lord? They said: Yes" [Surah al-A'raf: 172].
Hadith Referenceالسنن الكبرى للبيهقي / كتاب اللقطة / 12144