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 إِسْحَاقُ، أَخْبَرَنَا مُعْتَمِرٌ، قَالَ سَمِعْتُ حُمَيْدًا، يُحَدِّثُ عَنْ أَنَسٍ ـ رضى الله عنه ـ أَنَّ النَّبِيَّ صلى الله عليه وسلم كَانَ خَاتَمُهُ مِنْ فِضَّةٍ وَكَانَ فَصُّهُ مِنْهُ. وَقَالَ يَحْيَى بْنُ أَيُّوبَ حَدَّثَنِي حُمَيْدٌ سَمِعَ أَنَسًا عَنِ النَّبِيِّ صلى الله عليه وسلم.
Narrated Anas:
The ring of the Prophet (ﷺ) was of silver, and its stone was of silver too.
Reference: Sahih al-Bukhari 5870 In-book : Book 77, Hadith 87 USC-MSA web (English) : Vol. 7, Book 72, Hadith 759 (deprecated)
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