Dought & clear, - He did sujood al-tilaawah with the imam at the back of the mosque, then he came forward and joined the prayer. - (Prostration when reading Qur’aan and Prostration of thanksgiving)

A man entered the mosque – which was a large mosque – when the imam
was reciting, and when the imam reached a verse in which a prostration
is required (sujood al-tilaawah), he prostrated. The man thought that
if he went and joined the row he would not catch up withthat
prostration with the imam, so he prostrated while he wasstill in the
back of the mosque, then he stood up from the prostration and reached
the row and joined the prayer with the imam. Is what that man did
correct?.
Praise be to Allaah.
That is not prescribed in sharee'ah, unless he waslistening to the
recitation purposefully, in which case he could prostrate with the
imam even though he was not praying behind him. But if his intention
was to join the imam and pray behind him, then what he did was an
invalid action.
Shaykh Dr 'Abd al-Kareem al-Khudayr.