The Quran states very, very clearly not to uphold any other source of religious guidance besides the Quran. That means any and all other sources. The 'Medhi' is a hadith fabrication. There is no mention of any 'Mehdi' in the Quran, but there is mention of a messenger that will come after all scriptures have revealed for the purpose of confirming the Quran is, in fact, from GOD.
[Quran 3:81] When Allah made (His) covenant with the prophets, (He said): Behold that which I have given you of the Scripture and knowledge. And afterward there will come unto you a messenger, confirming that which ye possess. Ye shall believe in him and ye shall help him. He said: Do ye agree, and will ye take up My burden (which I lay upon you) in this (matter) ? They answered: We agree. He said: Then bear ye witness. I will be a witness with you.
There is a major misunderstanding that this verse applies to Muhammad. Muhammad was prophet and was given scripture. This verse specifically addresses the Prophets who were given scriptures. Thus, Muhammad was one of the prophets who took this covenant. As well, this verse says 'afterward' a messenger will come to confirm that which ye possess. When you read this verse, you are reading the Quran. Thus, what is in your possession is the Quran. Thus, it is the Quran that is being 'confirmed'.
Another reason for the misunderstanding is because of the mistranslating of Sura 33:40. Many believe it says Muhammad is the last messenger and last prophet, but the Quran is clearly saying otherwise. For example, Yusuf Ali translates it as follows:
[33:40 Yusuf Ali] Muhammad is not the father of any of your men, but (he is) the Apostle of God, and the Seal of the Prophets: and God has full knowledge of all things.
The arabic is very clear, 'rasoolallah wa Khaatimun Nabiyeen' (Messenger and final Prophet).
Rasool = Messenger
Allah = GOD
wa= and
Khaatimum = final
*Nabiyeen *= Prophet
As you can see very clearly that Muhammad IS the last Prophet, but NOT the last messenger as there is a messenger coming after the Quran has been revealed for the purpose of 'confirming' it (Quran 3:81). Thus, prophets bring scripture, messengers confirm scripture. Remember, the Quran tells us every community gets a messenger, some we know about, some we do not. If messengers and prophets were the same thing there would be thousands of scriptures circulating the world......but there are not.
Peace