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 يُوسُفَ، أَخْبَرَنَا مَالِكٌ، عَنْ عَبْدِ اللَّهِ بْنِ دِينَارٍ، عَنْ عَبْدِ اللَّهِ بْنِ عُمَرَ ـ رضى الله عنهما ـ أَنَّ رَجُلاً، ذَكَرَ لِلنَّبِيِّ صلى الله عليه وسلم أَنَّهُ يُخْدَعُ فِي الْبُيُوعِ، فَقَالَ إِذَا بَايَعْتَ فَقُلْ لاَ خِلاَبَةَ "
Narrated `Abdullah bin `Umar:
A person came to the Prophet (ﷺ) and told him that he was always betrayed in purchasing. The Prophet (ﷺ) told him to say at the time of buying, "No cheating."
Reference: Sahih al-Bukhari 2117 In-book : Book 34, Hadith 70 USC-MSA web (English) : Vol. 3, Book 34, Hadith 328 (deprecated)
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