Hadith.1172 - A question was asked about a man who enters the mosque while the congregation is already in prayer, and the Imam is ahead of him by one rak'ah. He pronounces the takbir, and then the Imam becomes unable to continue (due to illness or another reason) and takes the man’s hand (indicating him to lead), and he happens to be the nearest person to the Imam. Imam (as) said: "He should lead them in completing the prayer, then sit until they finish their tashahhud (final testimony). He should gesture to the right and left with his hand as a sign of completing the prayer (taslim) or wait until the prayer is completed by the congregation, after which he can complete what he missed of his own prayer."