In a four-rak'ah Fard such as Zuhr, Asr or Isha:
• if the first Sajdah has been completed when the congregation is established, complete the second rak'ah;
• give Salaam after 2;
• those 2 rak'ahs become voluntary prayer;
• then join the congregational Fard.
This preserves a valid two-rak'ah Nafl unit instead of discarding it unnecessarily.
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When Congregation Begins during an Individual Fard or Sunnah Prayer
In a four-rak'ah Fard such as Zuhr, Asr or Isha:
• if the first Sajdah has been completed when the congregation is established, complete the second rak'ah;
• give Salaam after 2;
• those 2 rak'ahs become voluntary prayer;
• then join the congregational Fard.
This preserves a valid two-rak'ah Nafl unit instead of discarding it unnecessarily.