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، حَدَّثَنَا اللَّيْثُ، حَدَّثَنِي ابْنُ الْهَادِ، عَنْ عَبْدِ اللَّهِ بْنِ خَبَّابٍ، عَنْ أَبِي سَعِيدٍ ـ رضى الله عنه ـ أَنَّهُ سَمِعَ النَّبِيَّ صلى الله عليه وسلم يَقُولُ لاَ تُوَاصِلُوا، فَأَيُّكُمْ إِذَا أَرَادَ أَنْ يُوَاصِلَ فَلْيُوَاصِلْ حَتَّى السَّحَرِ ". قَالُوا فَإِنَّكَ تُوَاصِلُ يَا رَسُولَ اللَّهِ. قَالَ " إِنِّي لَسْتُ كَهَيْئَتِكُمْ، إِنِّي أَبِيتُ لِي مُطْعِمٌ يُطْعِمُنِي وَسَاقٍ يَسْقِينِ "."
Narrated Abu Sa`id:
That he had heard the Prophet (ﷺ) saying, "Do not fast continuously (practice Al-Wisal), and if you intend to lengthen your fast, then carry it on only till the Suhur (before the following dawn)." The people said to him, "But you practice (Al-Wisal), O Allah's Messenger (ﷺ)!" He replied, "I am not similar to you, for during my sleep I have One Who makes me eat and drink."
Reference: Sahih al-Bukhari 1963 In-book : Book 30, Hadith 70 USC-MSA web (English) : Vol. 3, Book 31, Hadith 184 (deprecated)
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