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
The Prophet (ﷺ) said to me, "I have been informed that you pray all the nights and observe fast all the days; is this true?" I replied, "Yes." He said, "If you do so, your eyes will become weak and you will get bored. So fast three days a month, for this will be the fasting of a whole year, or equal to the fasting of a whole year." I said, "I find myself able to fast more." He said, "Then fast like the fasting of (the Prophet) David who used to fast on alternate days and would not flee on facing the enemy."
Reference: Sahih al-Bukhari 3419 In-book : Book 60, Hadith 91 USC-MSA web (English) : Vol. 4, Book 55, Hadith 630 (deprecated)
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