What happens to a mandatory prayer if the worshiper utters the Takbir while leaning into Ruku?

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If the Takbir for Ruku is uttered during the mandatory prayer while one is leaning into the bowing posture, the prayer is converted into a supererogatory (Nafl) act. This ruling stems from the inability of the act to occur in its mandatory form, but its potential to stand as a voluntary prayer, analogous to one who commences a mandatory prayer before its prescribed time has begun.