What should one do who has unintentionally replaced the opening supplication (Du'a al-Istiftah) in prayer with the supplication for expiation of an assembly (Du'a Kaffarat al-Majlis)? And is it permissible to seek forgiveness (Istighfar) during bowing (Ruku') along with saying "Subhana Rabbiyal Azim" (Glory be to my Lord, the Most Magnificent)?

Reciting the supplication for expiation of the مجلس (gathering) instead of the opening supplication does not invalidate the prayer, nor does it necessitate a prostration of forgetfulness (sujud (definition) sahwi). As for bowing (ruku'), it is legislated to seek forgiveness therein, based on the hadith (definition) of Aisha (may Allah be pleased with her), that the Messenger of Allah (peace and blessings of Allah be upon him) used to frequently say in his bowing and prostration: "Exalted are You, O Allah, our Lord, and with Your praise; O Allah, forgive me."