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 مُسَدَّدٌ، قَالَ حَدَّثَنِي يَحْيَى، عَنْ شُعْبَةَ، قَالَ حَدَّثَنِي قَتَادَةُ، عَنْ أَنَسٍ ـ رضى الله عنه ـ عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ لاَ تُوَاصِلُوا ". قَالُوا إِنَّكَ تُوَاصِلُ. قَالَ " لَسْتُ كَأَحَدٍ مِنْكُمْ، إِنِّي أُطْعَمُ وَأُسْقَى، أَوْ إِنِّي أَبِيتُ أُطْعَمُ وَأُسْقَى "."
Narrated Anas:
The Prophet (ﷺ) said, "Do not practice Al-Wisal (fasting continuously without breaking one's fast in the evening or eating before the following dawn)." The people said to the Prophet, "But you practice Al- Wisal?" The Prophet (ﷺ) replied, "I am not like any of you, for I am given food and drink (by Allah) during the night."
Reference: Sahih al-Bukhari 1961 In-book : Book 30, Hadith 68 USC-MSA web (English) : Vol. 3, Book 31, Hadith 182 (deprecated)
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