✅ Answer: Please refer to the following narrations:

1⃣عَنْ هِشَامِ بْنِ سَالِمٍ عَنْ سُلَیْمَانَ بْنِ خَالِدٍ قَالَ: سَأَلْتُ أَبَا عَبْداللَّهِ عَنْ رَجُلٍ دَخَلَ الْمَسْجِدَ فَافْتَتَحَ الصَّلَاةَ فَبَیْنَا هُوَ قَائِمٌ یُصَلِّی إِذْ أَذَّنَ الْمُؤَذِّنُ وَ أَقَامَ الصَّلَاةَ قَالَ فَلْیُصَلِّ رَکْعَتَیْنِ ثُمَّ یَسْتَأْنِفُ الصَّلَاةَ مَعَ الْإِمَامِ وَ لْتَکُنِ الرَّکْعَتَانِ تَطَوُّعاً.

📚 Al-Kafi, Vol. 3, p. 379

Sulaiman bin Khalid says: I asked Imam Sadiq (peace be upon him) about a person who enters the mosque, starts praying, and while they are standing in prayer, the muezzin calls the adhan and iqama for congregational prayer. The Imam said: They should pray two units of prayer (as a nafl) and then join the imam's congregational prayer, and these two units should be voluntary.

2⃣عَنْ مُحَمَّدِ بْنِ یَحْیَی عَنْ أَحْمَدَ بْنِ مُحَمَّدٍ عَنْ عُثْمَانَ بْنِ عِیسَی عَنْ سَمَاعَةَ قَالَ: سَأَلْتُهُ عَنْ رَجُلٍ کَانَ یُصَلِّی فَخَرَجَ الْإِمَامُ وَ قَدْ صَلَّی الرَّجُلُ رَکْعَةً مِنْ صَلَاةٍ فَرِیضَةٍ قَالَ إِنْ کَانَ إِمَاماً عَدْلًا فَلْیُصَلِّ أُخْرَی وَ یَنْصَرِفُ وَ یَجْعَلُهُمَا تَطَوُّعاً وَ لْیَدْخُلْ مَعَ الْإِمَامِ فِی صَلَاتِهِ کَمَا هُوَ

📚 Al-Kafi, Vol. 3, p. 380

Sama'a says: I asked Imam Sadiq (peace be upon him) about a person who is praying and then the imam of the congregation arrives, and they have already prayed one unit of the obligatory prayer. The Imam said: If the imam of the congregation is just, they should pray one more unit, say the tasleem (salam), consider it a nafl (voluntary prayer), and join the imam in his prayer.

✅ Based on the above narrations, if a person starts their prayer individually and then a congregation with a just imam is established, they can complete their prayer as two units of nafl and join the congregational prayer.

✅ It is observed that breaking or abandoning the prayer to join the congregational prayer is not allowed, and the person can only complete their prayer as two units of nafl and then join the congregational prayer.

اللَّهُمَّ صَلِّ عَلَی مُحَمَّدٍ وَ آلِ مُحَمَّدٍ وَ عَجِّلْ لِوَلِیِّکَ الْفَرَجَ وَ الْعَافِیَةَ وَ النَّصْر

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