In a four-rak'ah Fard, if the person has completed 2 rak'ahs and stood for the third when congregation begins:
• if the third-rak'ah Sajdah has not yet been performed, the correct view stated here allows ending the prayer while standing with one Salaam and joining the congregation;
• the completed first 2 rak'ahs stand as voluntary prayer.
The presence or absence of the third-rak'ah Sajdah is therefore decisive.
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When Congregation Begins during an Individual Fard or Sunnah Prayer
In a four-rak'ah Fard, if the person has completed 2 rak'ahs and stood for the third when congregation begins:
• if the third-rak'ah Sajdah has not yet been performed, the correct view stated here allows ending the prayer while standing with one Salaam and joining the congregation;
• the completed first 2 rak'ahs stand as voluntary prayer.
The presence or absence of the third-rak'ah Sajdah is therefore decisive.