For example: a man forgets to say, “Glory be to my Lord, the Magnificent,” yet he has not missed any part of the prayer; the prostration for forgetfulness is therefore waived for him.
But if he has missed part of the prayer and must complete it after his imam has said the salam, then he must perform the prostration for forgetfulness if he commits an error that requires it, because by prostrating he does not differ from his imam.
For example: a man forgets to say, “Glory be to my Lord, the Magnificent,” while bowing, and he joined the imam in the second rak‘ah. This omission requires him to perform the prostration for forgetfulness, because he has omitted an obligation and has missed part of the prayer. Thus, when he stands and performs the rak‘ah he missed, he must prostrate for forgetfulness on account of having omitted an obligation, because by prostrating he does not differ from the imam: he is praying independently while making up what he missed of the prayer.
His statement, “except in conjunction with his imam,” means: unless his prostration is in following his imam, in which case it is obligatory for him, whether he himself has forgotten or not. Thus, when the imam prostrates, the follower must follow him, due to the general meaning of the Messenger of Allah’s statement, peace and blessings be upon him: “The imam has only been appointed to be followed, so do not differ from him.”
For example: the imam omits saying, “Glory be to my Lord, the Most High,” while prostrating, while the follower is unaware of this, because the imam does not recite the glorification aloud. Then, when he intends to say the salam, he performs two prostrations for having omitted the obligatory glorification. The follower has omitted none of the obligations or pillars, yet he must prostrate in following the imam, just as he must sit in his first rak‘ah if he joined the imam in the second rak‘ah, even though this is not a point at which he would normally sit; rather, he sits in following the imam. This applies when the imam’s prostration is before the salam, because the imam