If 3 rak'ahs of a four-rak'ah Fard have already been completed:
• complete the fourth rak'ah and finish the Fard;
• after that, one may join the congregation with the intention of Nafl in prayers where such repetition is allowed;
• the source excludes Asr from this post-Fard voluntary joining because voluntary prayer after Asr is restricted.
At this late stage the individual Fard is completed rather than cut.
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When Congregation Begins during an Individual Fard or Sunnah Prayer
If 3 rak'ahs of a four-rak'ah Fard have already been completed:
• complete the fourth rak'ah and finish the Fard;
• after that, one may join the congregation with the intention of Nafl in prayers where such repetition is allowed;
• the source excludes Asr from this post-Fard voluntary joining because voluntary prayer after Asr is restricted.
At this late stage the individual Fard is completed rather than cut.