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
Anas was asked, "Did the Prophet (ﷺ) wear a ring?" Anas said, "Once he delayed the: `Isha' prayer till midnight. Then he came, facing us ..... as if l am now Looking at the glitter of his ring ..... and said, "The people have offered their prayers and slept but you have been in prayer as you have been waiting for it."
Reference: Sahih al-Bukhari 5869 In-book : Book 77, Hadith 86 USC-MSA web (English) : Vol. 7, Book 72, Hadith 758 (deprecated)
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