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
Allah's Messenger (ﷺ) was informed that I have said: "By Allah, I will fast all the days and pray all the nights as long as I live." On that, Allah's Messenger (ﷺ) asked me. "Are you the one who says: 'I will fast all the days and pray all the nights as long as I live?' " I said, "Yes, I have said it." He said, "You cannot do that. So fast (sometimes) and do not fast (sometimes). Pray and sleep. Fast for three days a month, for the reward of a good deed is multiplied by ten time, and so the fasting of three days a month equals the fasting of a year." I said, "O Allah's Messenger (ﷺ)! I can do (fast) more than this." He said, "Fast on every third day. I said: I can do (fast) more than that, He said: "Fast on alternate days and this was the fasting of David which is the most moderate sort of fasting." I said, "O Allah's Messenger (ﷺ)! I can do (fast) more than that." He said, "There is nothing better than that."
Reference: Sahih al-Bukhari 3418 In-book : Book 60, Hadith 90 USC-MSA web (English) : Vol. 4, Book 55, Hadith 629 (deprecated)
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