Q: Coming in the Masjid a little late, I found the brothers in ruku, I was afraid I would miss the rak’ah and so I made ruku before I made it to the row. Was that valid?
A: If you made ruku behind the row due to some excuse e.g. fear of missing the rak’ah, it is valid so long as you joined the row before the Imam prostrated. This is an exception afforded to those with an excuse and not acceptable as a regular practice. Source: Sharh al-Muntaha