أَلَسْتُ بِرَبِّكُمْ قَالُوا بَلَى شَهِدْنَا
“Am I not your Lord?” They said, “Yes indeed; we bear witness.”
The question أَلَسْتُ بِرَبِّكُمْ concerns Allah’s Lordship. بَلَى affirms a statement posed in a negative question. The descendants thereby acknowledged that Allah is their Lord.