One who joined the prayer late leading another
Question

                A man entered the mosque after the imaam and congregation had said the salaam, but he found someone who had joined the prayer late and who was completing his prayer, so he stood beside him to make that man an imaam for him and so that he might attain the reward for praying in jamaa’ah. Is that permissible for him or not? And is the prayer that this man did with the one who had joined the prayer late valid?
Praise be to Allah.
    Shaykh ‘Abd al-‘Azeez ibn Baaz (may Allaah have mercy on him) 
    said: “If a person comes late to the mosque and the people have already 
    prayed, and he finds someone who joined the prayer late still praying, it is 
    prescribed for him to pray with him and to stand to the right of the one who 
    had joined the prayer late, seeking the reward of praying in jamaa’ah 
    (congregation). The one who had joined the prayer late should then intend to 
    lead the other man. There is nothing wrong with that according to the more 
    correct of the two scholarly opinions. Similarly, if he finds a man praying 
    alone after the imaam has said the salaam, it is prescribed for him to pray 
    with him and to stand on his right, seeking the reward of praying in 
    jamaa’ah (congregation). When the one who had joined the prayer late or the 
    one who was praying alone says the salaam, then the one who came in last 
    should stand up and complete his prayer, because of the general meaning of 
    the evidence which speaks of the virtue of praying in congregation, and 
    because it was proven that when the Prophet  (peace and blessings of 
    Allaah be upon him) saw a man who had entered the mosque after the prayer 
    was over, he said, “If anyone wants to do this man a favour, let him pray with him.” (Narrated by Abu Dawood, 574..