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 نَافِعٍ، عَنْ عَبْدِ اللَّهِ بْنِ عُمَرَ ـ رضى الله عنهما ـ قَالَ نَهَى رَسُولُ اللَّهِ صلى الله عليه وسلم عَنِ الْوِصَالِ. قَالُوا إِنَّكَ تُوَاصِلُ. قَالَ إِنِّي لَسْتُ مِثْلَكُمْ، إِنِّي أُطْعَمُ وَأُسْقَى "
Narrated `Abdullah bin `Umar:
Allah's Messenger (ﷺ) forbade Al-Wisal. The people said (to him), "But you practice it?" He said, "I am not like you, for I am given food and drink by Allah."
Reference: Sahih al-Bukhari 1962 In-book : Book 30, Hadith 69 USC-MSA web (English) : Vol. 3, Book 31, Hadith 183 (deprecated)
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