✅Answer: Please pay attention to the following narrations:

1 عَنْ أَبِي بَصِيرٍ قَالَ: قُلْتُ لِأَبِي عَبْدِ اللَّهِ أُصَلِّي ثُمَّ أَدْخُلُ الْمَسْجِدَ فَتُقَامُ الصَّلَاةُ وَ قَدْ صَلَّيْتُ فَقَالَ صَلِّ مَعَهُمْ يَخْتَارُ اللَّهُ أَحَبَّهُمَا إِلَيْهِ

📚 Tahdheeb, vol. 3, p. 270

📚 Al-Kafi, vol. 3, p. 379

Translation: Abu Basir said: I said to Imam Sadiq, peace be upon him: I pray and then I enter the mosque, and the (congregational) prayer is established while I have already prayed; The Imam said: Pray with them, and Allah will choose the one of the two prayers that is more beloved to Him.

2 عَنْ عَمَّارٍ قَالَ: سَأَلْتُ أَبَا عَبْدِ اللَّهِ عَنِ الرَّجُلِ يُصَلِّي الْفَرِيضَةَ ثُمَّ يَجِدُ قَوْماً يُصَلُّونَ جَمَاعَةً أَ يَجُوزُ لَهُ أَنْ يُعِيدَ الصَّلَاةَ مَعَهُمْ قَالَ نَعَمْ وَ هُوَ أَفْضَلُ قُلْتُ فَإِنْ لَمْ يَفْعَلْ قَالَ لَيْسَ بِهِ بَأْسٌ.

📚 Tahdheeb, vol. 3, p. 50

Translation: Ammar said: I asked Imam Sadiq, peace be upon him, about a person who performs the obligatory prayer and then finds a group that is praying in congregation: Should he repeat his prayer with them? He said: Yes, and this is better and more virtuous. I said: What if he does not do it? He said: There is no problem with that.

3 عَنْ حَفْصِ بْنِ الْبَخْتَرِيِّ عَنْ أَبِي عَبْدِ اللَّهِ فِي الرَّجُلِ يُصَلِّي الصَّلَاةَ وَحْدَهُ ثُمَّ يَجِدُ جَمَاعَةً قَالَ يُصَلِّي مَعَهُمْ وَ يَجْعَلُهَا الْفَرِيضَةَ.

📚 Al-Kafi, vol. 3, p. 379

Translation: Imam Sadiq, peace be upon him, said about a person who prays individually and then finds a congregation: He prays with them and considers it the obligatory prayer.

4 مُحَمَّدُ بْنُ عَلِيِّ بْنِ الْحُسَيْنِ بِإِسْنَادِهِ عَنْ هِشَامِ بْنِ سَالِمٍ عَنْ أَبِي عَبْدِ اللَّهِ أَنَّهُ قَالَ: فِي الرَّجُلِ يُصَلِّي الصَّلَاةَ وَحْدَهُ ثُمَّ يَجِدُ جَمَاعَةً قَالَ يُصَلِّي مَعَهُمْ وَ يَجْعَلُهَا الْفَرِيضَةَ إِنْ شَاءَ.

📚 Faqih, vol. 1, p. 384

Translation: Imam Sadiq, peace be upon him, said about a person who prays individually and then finds a congregation: If he wants, he prays with them and considers it the obligatory prayer.

5 عَنْ إِسْحَاقَ بْنِ عَمَّارٍ قَالَ: قُلْتُ لِأَبِي عَبْدِ اللَّهِ تُقَامُ الصَّلَاةُ وَ قَدْ صَلَّيْتُ فَقَالَ صَلِّ وَ اجْعَلْهَا لِمَا فَاتَ

📚 Tahdheeb, vol. 3, p. 50

Translation: Ishaq ibn Ammar said: I said to Imam Sadiq, peace be upon him: The (congregational) prayer is established while I have already prayed; The Imam said: Pray and consider it for the missed (prayers).

✅It is clearly observed in the above traditions that if a person has prayed individually and then the same prayer is established in congregation in a mosque or elsewhere, if he wishes, he can pray with them and consider it the same obligatory prayer. Although he has the choice of doing it or not, according to the Imam's saying, peace be upon him, it is more virtuous, and Allah will choose the one of the two prayers that is more beloved to Him.

✅According to tradition 5 as well, he can consider this second prayer as a makeup prayer (qada').

RepeatingPrayer, Individual Prayer, Makeup Prayer, Praying in Congregation