The emergence of the Mahdi
Question

Is it told in the Quran when the Mahdi will come to save the muslims? .
Praise be to Allah.The Muslim must understand that evidence from both the Qur’aan and Sunnah are of the same level and must be followed equally. Both the Qur’aan and Sunnah are Revelation (Wahy) from Allah and must be followed. Allah says concerning His Prophet Muhammad (peace and blessings of Allah be upon him) (interpretation of the meaning): 
“Nor does he speak of (his own) desire.
It is only a Revelation revealed” [al-Najm 53:4-3]
It was narrated from al-Miqdaam ibn Ma’dikarib that the Messenger of Allah (peace and blessings of Allah be upon him) said: “Beware! I have been given the Qur’aan and something like it, yet the time is coming when a man replete on his couch will say: ‘Keep to the Qur’aan; what you find in it to be permissible treat as permissible, and what you find in it to be prohibited treat as prohibited.’” 
(Narrated by Abu Dawood, 4606; classed as saheeh by al-Albaani – may Allah have mercy on him – in Saheeh Sunan Abi Dawood, 3848). 
Secondly: 
Allah has enjoined that His Messenger is to be obeyed independently of the Qur’aan. Allah says (interpretation of the meaning): 
“O you who believe! Obey Allah and obey the Messenger (Muhammad), and those of you (Muslims) who are in authority” [al-Nisa’ 4:59]
“And whatsoever the Messenger (Muhammad) gives you, take it; and whatsoever he forbids you, abstain (from it)” [al-Hashr 59:7]
Thirdly: 
It is not narrated in either the Qur’aan or the Sunnah precisely when the Mahdi will emerge, but he will emerge at the end of time. However we must point out the following: 
1-The emergence of the Mahdi will be one of the last minor signs of the Hour.
2- Many people claimed that the Mahdi has emerged or will emerge to support their personal aims and misguided beliefs, such as the Qadianis (Ahmadis), Baha’is, Shi’ah and other deviant groups. This has led some later scholars to reject the ahaadeeth which speak of the Mahdi or to misinterpret them by saying that what is meant by the Mahdi is the descent of ‘Eesa ibn Maryam (Jesus son of Mary – peace be upon him) at the end of time. Some of them quote a hadeeth that is attributed to the Prophet (peace and blessings of Allah be upon him) which says, “There is no Mahdi apart from ‘Eesa ibn Maryam.”
This hadeeth is da’eef (weak) and cannot be soundly attributed to the Prophet (peace and blessings of Allah be upon him). See al-Silsilah al-Da’eefah, 1/175, by al-Albaani  (may Allah have mercy on him).
3-Many scholars have written books confirming that the Mahdi will emerge, and describing that as part of the belief (‘aqeedah) of the Muslim. For example: al-Haafiz Abu Na’eem, Abu Dawood, Abu Katheer, al-Sakhaawi, al-Shawkaani, and others.
4-It is proven in the Sunnah that the Mahdi will meet ‘Eesa ibn Maryam (peace be upon him) and that ‘Eesa will pray behind him.
Muslim (156) narrated that Jaabir ibn ‘Abd-Allah (may Allah be pleased with him) said: “I heard the Prophet (peace and blessings of Allah be upon him) say, ‘A group of my ummah will continue to fight for the truth and prevail until the Day of Resurrection. Then ‘Eesa ibn Maryam will come down and their leader will say, “Come and lead us in prayer.” But he will say, “No, you are leaders of one another” – as an honour to this ummah.’”
The leader mentioned in this hadeeth is the Mahdi. According to a report narrated by Abu Na’eem and al-Haarith ibn Usaamah, “Their leader the Mahdi will say…” Ibn al-Qayyim said, “Its isnaad is jayyid (good).”
5-The Muslim should not wait for the emergence of the Mahdi, rather he has to strive hard and devote his efforts to support Islam, and to do whatever he can for the religion. He should not hope and wait for the emergence of the Mahdi or anyone else, rather he should correct himself, his family and the people around him, so that when he meets Allah he meet Him having done his best.
See Al-Mahdi Haqeeqah wa laa Khuraafah by Shaykh Muhammad ibn Ismaa’eel.