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
حَدَّثَنَا عَبْدُ اللَّهِ بْنُ مُحَمَّدٍ، حَدَّثَنَا عَبْدُ الرَّزَّاقِ، أَخْبَرَنَا مَعْمَرٌ، عَنْ هَمَّامٍ، عَنْ أَبِي هُرَيْرَةَ ـ رضى الله عنه ـ عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ خُفِّفَ عَلَى دَاوُدَ ـ عَلَيْهِ السَّلاَمُ ـ الْقُرْآنُ، فَكَانَ يَأْمُرُ بِدَوَابِّهِ فَتُسْرَجُ، فَيَقْرَأُ الْقُرْآنَ قَبْلَ أَنْ تُسْرَجَ دَوَابُّهُ، وَلاَ يَأْكُلُ إِلاَّ مِنْ عَمَلِ يَدِهِ " رَوَاهُ مُوسَى بْنُ عُقْبَةَ عَنْ صَفْوَانَ، عَنْ عَطَاءِ بْنِ يَسَارٍ، عَنْ أَبِي هُرَيْرَةَ، عَنِ النَّبِيِّ صلى الله عليه وسلم
Narrated Abu Huraira:
The Prophet (ﷺ) said, "The reciting of the ZAbu r (i.e. Psalms) was made easy for David. He used to order that his riding animals be saddled, and would finish reciting the ZAbu r before they were saddled. And he would never eat except from the earnings of his manual work."
Reference: Sahih al-Bukhari 3417 In-book : Book 60, Hadith 89 USC-MSA web (English) : Vol. 4, Book 55, Hadith 628 (deprecated)
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