What is the ruling on the follower completing the prayer before the imam?

Question:
The questioner, Abu Mu'adh, says: Some people complete the prayer with the imam, while others complete it after the imam's first closing salam; is their prayer valid, and what is the ruling in this case?

Answer:
It is obligatory to complete the prayer after the imam; the Prophet (peace and blessings be upon him) commanded that one not complete the prayer except after the imam's salam, so it is not permissible for the follower to complete the prayer except after his imam's salam. If he intentionally completes the prayer before his imam's salam, or before the second closing salam, his prayer is invalid, and he must repeat it if it was an obligatory prayer. As for if he was inattentive, no, if he was inattentive, he returns, intends the prayer, then completes the prayer after his imam, and it does not harm him if he was inattentive.