What is the ruling on the prayer of one who intended to shorten it (qasr) then completed it with the Imam who completed the prayer, or sat for the tashahhud after the second rak'ah then said the taslim with the Imam?

The correct view is with those who followed the Imam and completed their prayer with him, and this is obligatory upon the ma'mum (follower) if he prays behind someone who is completing the prayer, even if he realizes this during the prayer. Some jurists have explicitly addressed this issue, such as Al-Nawawi, who stated that whoever doubts whether his Imam is completing the prayer or is forgetful, should complete it. Similarly, Al-Bahuti Al-Hanbali said that the ma'mum is obliged to complete the prayer if the resident Imam appoints a successor.

This indicates that changing the intention from shortening the prayer (qasr) to completing it (itmam) in this case is not harmful; rather, it is obligatory. Therefore, whoever did not follow the Imam and did not complete his prayer has erred, and must repeat that prayer.

As for the view of Ibn Hazm, it differs from the view of the majority. He believes that shortening the prayer is obligatory even if the Imam is a resident, and he argued with generalities that do not support his claim, and which, according to the majority, are restricted by what was authentically reported from Ibn Abbas, that the Sunnah is for the worshiper to complete his prayer behind his Imam. The Companions rebuked Uthman for completing his prayer during travel but followed him in it, and it has not been reported that any of them shortened the prayer behind him.