Must the congregant perform the prostration of forgetfulness (sujood as-sahw) pre-emptively or post-emptively—or is it not incumbent upon him at all—if he fell behind his imam by an entire rak'ah, then realized the situation after the imam's salutation?

If the congregant (ma'mum) lags behind his imam by a full raka'ah without a valid excuse, his prayer is invalid and must be repeated. However, if there is a legitimate excuse (such as forgetting), he continues with his imam and makes up the raka'ah after the imam's taslim.