What is the ruling on joining the row after bowing without a valid excuse or fear of missing the prayer altogether?

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There are two opinions regarding performing this act without an excuse or fear of missing the prayer. One view holds that it is sufficient because if it were invalid generally, it would also be invalid when there is an excuse, like missing the entire rak'ah. The second view maintains it is invalid because the default ruling is prohibition, as the person misses what is missed by missing the rak'ah, and the allowance was made only for the excuse mentioned in Abu Bakra's Hadith; thus, in the absence of an excuse, the original ruling of invalidity applies.