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
(One day) I sat along with Shaiba on the chair inside the Ka`ba. He (Shaiba) said, "No doubt, `Umar sat at this place and said, 'I intended not to leave any yellow (i.e. gold) or white (i.e. silver) (inside the Ka`ba) undistributed.' I said (to `Umar), 'But your two companions (i.e. The Prophet (ﷺ) and Abu Bakr) did not do so.' `Umar said, They are the two persons whom I always follow.' "
Reference: Sahih al-Bukhari 1594 In-book : Book 25, Hadith 80 USC-MSA web (English) : Vol. 2, Book 26, Hadith 664 (deprecated)
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