If somebody is leading the Salat (prayer) and (in the meanwhile) the first (usual) Imam comes, the Salat is valid whether the former retreats or does not retreat
حَدَّثَنِي سُلَيْمَانُ، حَدَّثَنَا شُعْبَةُ، عَنِ الْوَلِيدِ،. وَحَدَّثَنِي عَبَّادُ بْنُ يَعْقُوبَ الأَسَدِيُّ، أَخْبَرَنَا عَبَّادُ بْنُ الْعَوَّامِ، عَنِ الشَّيْبَانِيِّ، عَنِ الْوَلِيدِ بْنِ الْعَيْزَارِ، عَنْ أَبِي عَمْرٍو الشَّيْبَانِيِّ، عَنِ ابْنِ مَسْعُودٍ ـ رضى الله عنه ـ أَنَّ رَجُلاً، سَأَلَ النَّبِيَّ صلى الله عليه وسلم أَىُّ الأَعْمَالِ أَفْضَلُ قَالَ الصَّلاَةُ لِوَقْتِهَا، وَبِرُّ الْوَالِدَيْنِ، ثُمَّ الْجِهَادُ فِي سَبِيلِ اللَّهِ "
Narrated Ibn Mas`ud:
A man asked the Prophet (ﷺ) "What deeds are the best?" The Prophet (ﷺ) said: "(1) To perform the (daily compulsory) prayers at their (early) stated fixed times, (2) to be good and dutiful to one's own parents, (3) and to participate in Jihad in Allah's Cause."
Reference: Sahih al-Bukhari 7534 In-book : Book 97, Hadith 159 USC-MSA web (English) : Vol. 9, Book 93, Hadith 625 (deprecated)
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