the imam, not knowing that this is prohibited and not knowing that he must return until the imam catches up with him, then his prayer is valid.
“If he bows and rises before his imam bows, knowingly and deliberately, his prayer is invalid. If he is ignorant or forgetful, only that rak‘ah is invalid. And if he bows and rises before the imam bows, then prostrates before the imam rises, his prayer is invalid—except in the case of one who is ignorant or forgetful, in which case he makes up that rak‘ah.”
His statement, “If he bows and rises before his imam bows, knowingly and deliberately, his prayer is invalid,” means that if he bows and rises before his imam bows, his prayer is invalid, because he has preceded the imam by the pillar of bowing. A person is not considered to have preceded the imam by a pillar unless he moves on from it to the pillar that follows it. Thus, if he bows and the imam catches up with him while he is still bowing, he is not considered to have preceded the imam by a pillar. Rather, we say that he preceded the imam in reaching the pillar, for the pillar in which the imam catches up with him is not considered a pillar by which he preceded him; rather, it is one to which he preceded him.
His statement, “If he is ignorant or forgetful, only that rak‘ah is invalid,” means that if he bows and rises before his imam, out of ignorance or forgetfulness, only the rak‘ah in which this precedence occurred is invalid. He must therefore make it up after the imam gives the salām.
In summary, if he precedes the imam by the pillar of bowing—by bowing and rising before the imam has bowed—then, if he did so deliberately, his prayer is invalid. If he did so out of ignorance or forgetfulness, only that rak‘ah is invalid, because he did not follow his imam in that bowing. He is therefore like one who did not catch the imam in it and thus missed that rak‘ah. But if he performs it after his imam, his rak‘ah is valid.
His statement, “If he bows and rises before the imam bows, then prostrates before the imam rises, his prayer is invalid—except in the case of one who is ignorant or forgetful, in which case he makes up that rak‘ah.”