Ruling on an additional Rak`ah added to the Salah with regard to latecomer

If the Imaam made a mistake and added an extra Rakah to the prayer, the latecomers should not count this extra Rakah. For example: If the Imaam forgot and added a fifth Rakah to the Dhuhr prayer, and there comes a man to the Masjid who missed the first two Rakaat, but he prayed three Rakaat with the Imaam knowing that the Imaam made an addition, in this case, he should not count this extra Rakah and make up for what he has missed.

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Shaikh Ibn Baaz رحمه الله said:

❝According to the more correct of the two opinions of scholars, if the Imaam unintentionally adds an extra Rakah, then the one joining him later in Salaah does not count it since it was done unintentionally by the Imaam. The person must complete the Salaah by performing (whatever he has missed without counting this extra Rakah) and then offer Sujood-ul-Sahw (Prostration of Forgetfulness) if he has not done so with the Imaam. If so, this suffices him. May Allaah grant us success.❞

[Majmoo` Fataawa of Shaikh Ibn Baaz (30/14-15)]