A traveller prayed the full prayer (itmam)

Question

If a traveller, for whom qasr was wajib, is the imam and leads resident (muqim) followers in the full four rak‘as, will the prayer of the resident followers be valid or not? In al-Durr al-Mukhtar it is written that it will not, and the reason given is this: only two rak‘as were wajib upon the traveller; of the four rak‘as he prayed, two became fard and the remaining two will be nafl.

Answer

The matter is clear; had you explained the reason for your confusion, it would have become known [where it lies]. When the traveller imam sat [for the qa‘da] after two rak‘as and completed four, two became fard and two nafl; the prayer of those followers who were residents was not valid in the last two rak‘as, because the following of one praying fard behind one praying nafl…

Source: Fatawa Mahmudiyya

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