The Ruling on the Follower Praying Behind an Imam Who Completes the Prayer

Question:
May Allah reward you. The Honorable Shaykh says: I was late and stood up for the 'Asr prayer, and I prayed 'Asr behind an imam. I intended to shorten the prayer, but the imam completed it. What is the ruling in this case?

Answer:
The ruling is that you complete the prayer following the imam if the imam completes it, whether he is a traveler or a resident. The resident is required to complete it, and the traveler is permitted to shorten it, but if he completes it, then those behind him are required to complete it. Did Abu [H] ask about his imam? The Prophet (peace and blessings be upon him) said: "The imam is appointed to be followed, so do not differ with him." It is not permissible to shorten the prayer, for that would be like differing with him.
As for the reverse, where the follower says the closing salam while the imam shortens the prayer, this is permissible. You pray behind someone who shortens the prayer, and then when he says the salam, you stand up and complete the prayer for yourself, provided that shortening is not permissible, as the Prophet (peace and blessings be upon him) did when he prayed with them. He said, “O people of Makkah, we are a people who have been prevented [from completing the journey], so complete the prayer for yourselves.” When the Prophet (peace and blessings be upon him) said the salam, they stood up and completed the prayer for themselves. The conclusion is that a traveler who prays behind a resident or someone who completes the prayer does not have [the option to shorten]. And if a resident prays behind a traveler who shortens the prayer, the resident completes the prayer. There is no harm in this. Yes. When he prayed with them, he said, “O people of Makkah, we are a people who have been prevented [from completing the journey], so complete the prayer for yourselves.” When the Prophet (peace and blessings be upon him) said the salam, they stood up and completed the prayer for themselves. The conclusion is that a traveler who prays behind a resident or someone who completes the prayer does not have [the option to shorten]. And if a resident prays behind a traveler who shortens the prayer, the resident completes the prayer. There is no harm in this. Yes.