Is believing in Al-Mahdi part of Islamic faith or not?

We believe that al- Mahdi will emerge at the end of the time and his appearance will increase the believers' belief in Prophet Muhammed (peace and blessings be upon him and his family) since he informed us of his appearance.

Al-Mahdi (peace be upon him) is not anticipated by the sunnis and there is no such command in Islamic law either on an individual basis or to the community as a whole. The completion of religion and the implementation of its rulings are not contingent upon the appearance of the Mahdi—Islam is Allah's last word to the worlds and the last covenant which He has chosen for mankind. Allah the Almighty says,
This day have I perfected your religion for you, completed my favor upon you, and have chosen for you Islam as your religion. [Al-Ma`ida, 3).

Islam is the religion of Allah which He has made flexible to encompass factors of change. He has therefore commanded His slaves to implement its rulings at all times and conditions without awaiting the Mahdi to do this and without making their acts contingent upon the emergence of a particular individual of whom they have been foretold. The purport of the hadiths on the subject of Al-Mahdi and the ultimate aim of knowing news of him or of others whom Prophet Mohammed said will emerge at the end of times, is belief in words of Prophet Mohammed, the infallible (peace and blessings be upon him and his family) and following the guidance of Al-Mahdi once he emerges. If any of this comes to pass, it will be considered another Prophetic miracle testifying to the validity of the words of the Prophet, the truthful one (peace be upon him and upon his family). Therefore, when people witness such a miracle, they will say, "This is what Allah and His Messenger have promised us. Allah and His Messenger have certainly told the truth". This will only increase the faith of Muslims and they will surrender to their religion.