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
Sa`d bin Abi Waqqas said, "The Prophet (ﷺ) passed wet hands over his Khuffs (socks made from thick fabric or leather)." `Abdullah bin `Umar asked `Umar about it. `Umar replied in the affirmative and added, "Whenever Sa`d narrates a Hadith from the Prophet, there is no need to ask anyone else about it."
Reference: Sahih al-Bukhari 202 In-book : Book 4, Hadith 68 USC-MSA web (English) : Vol. 1, Book 4, Hadith 201 (deprecated)
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