The Honorable Abdullah bin zubair saw a person making dua before finishing up his prayer with his hands raised in the last sitting of his prayer. He approached him and told him that it is sunna to do after finishing the prayer. This narration is a proof that the prophet peace be upon him use to do this.
The Honorable Abbas has a famous narration, wherein he stated that the prophet peace be upon him had given him a complete explanation of the prayer. He said that look, manifesting humility, poverty, helplessness, and servant hood is the prayer. After that, you should raise your hands and call on to him saying ‘ya rabbi, ya rabbi’ and he will answer your prayers.
Basically, there are other narrations reported by great companions like the Honorable Muaz, Anas, and Abu Huraira, which confirm the act as sunna.
There are some prayers that people should always recite after finishing the prayer. For example, “Rabbana Atina fid dunya hasant” etc. people should not forget to read these prayers after finishing their obligatory prayers. It is not an innovation to do such a thing rather it is recommended. A person can do this in congregation or individually or he can also say amen to the prayers of the leader. He can finish before the imam or after him.