Abu Sa'id al-Khudri told how a man came when God’s Messenger had already prayed, and he said, “Will no one give alms* to this man and pray along with him?” Thereupon a man got up and prayed along with him.
* This is a literal translation. The idea is that by joining with him he increased the reward the man would receive for his prayer, as prayer in company has a much greater reward than prayer alone.
Tirmidhi and Abu Dawud transmitted it.