Numerous traditions have been narrated from the Prophet (ṣ) and each one of the Imams (‘a) in regards to the birth, occultation, reappearance, global revolution, as well as other particularities of Imam Mahdi (‘a).
Numerous traditions have been narrated from the Prophet (ṣ) and each one of the Imams (‘a) in regards to the birth, occultation, reappearance, global revolution, as well as other particularities of Imam Mahdi (‘a). Years before his birth, many of the characteristics and particularities of Imam Mahdi (‘a) were foretold; these foretellings included: that he will be from the descendents of the Prophet (ṣ), he will be from the descendents of Fatimah (‘a), he will be from the lineage of Imam Husayn (‘a), and that he will fill the earth with justice through his global revolution. There are only a few subjects which have so many related traditions.
First, we will go over the number of traditions narrated from the Imams (‘a) about Imam Mahdi (‘a), and then we will review the books written by the scholars in regards to him. Afterwards, we will review the traditions narrated from the Prophet of Islam (ṣ) through the books of the Ahl al-Sunnah, as well as the books which their scholars have written in regards to Imam Mahdi (‘a).
It is worth mentioning here, that although the times of each of the Imams (‘a) had their own particularities and that each of the Imam’s (‘a) attention were naturally focused towards the critical issues of their day, that still, in spite of these issues, the subject of Imam Mahdi (‘a) was something which they all mentioned. In various times and places, the Imams (‘a) would speak about Imam Mahdi (‘a), and there are numerous traditions from each of the Imams (‘a) on this subject. For example:
1- 51 traditions from Imam ‘Ali (‘a)
2- 5 traditions from Imam Hasan (‘a)
3- 14 traditions from Imam Husayn (‘a)
4- 11 traditions from Imam Zayn al-’Abidīn (‘a)
5- 63 traditions from Imam Baqir (‘a)
6- 124 traditions from Imam Sadiq (‘a)
7- 6 traditions from Imam Mūsā ibn Ja’far (‘a)
8- 19 traditions from Imam Riḍa (‘a)
9- 6 traditions from Imam Jawad (‘a)
10- 6 traditions from Imam Hadi (‘a)
11- 22 traditions from Imam Hasan al-’Askari (‘a)
As an example, we will narrate several of these hadiths below:
A- Amīr al-Mu’minīn (‘a) has said: The Prophet of Islam (ṣ) said: The world will not come to an end until a man from the lineage of Husayn (‘a) takes control of the affairs of my nation and fills the world with justice, much like it had been filled with oppression.
In addition, the Nahj al-Balāghah is filled with the words of Imam ‘Ali (‘a) on this subject. For example, he is quoted as saying: The world is much like an unruly camel that does not allow (anyone) to drink its milk (and goes towards its own child); after this unruliness, it will (then) incline towards us. God has said: ‘And We wished to be gracious to those who were being depressed in the land, to make them leaders (in faith) and make them heirs’
B- Husayn ibn ‘Ali (‘a) has said: If there is only one day left in the existence of this earth, God will lengthen that day, in order that a man from my lineage will revolt and fill the world with justice and equity, much like it had been filled with oppression and tyranny. I heard the Prophet of God (ṣ) saying this.
C- Imam Jawad (‘a) is quoted as saying to ‘Abdul ‘Aẓīm Ḥasanī: Our Qā’im is the Mahdi Muntaẓar (‘a), who must be awaited during the time of his occultation and must be obeyed during the time of his reappearance. He will be my third descendent. I swear by the Lord who appointed Muhammad to Prophethood and who made the Imamate particular to our family, if there is only one day left in the life of this world, God will lengthen that day in duration until the Mahdi reappears and fills the earth with justice and equity, much like it had been filled with oppression and tyranny (before). God will accomplish his work in one night, much like he accomplished the work of Mūsā, the ‘Kalīmallah’: He went to bring a firebrand for his wife, but he returned with the rank of prophethood. Imam Jawad (‘a) then added: The most superior action of our Shia is the awaiting of his reappearance and revolt. 
 Amīnī, Ibrāhīm, The One Who Will Administer Justice Throughout The World, Fourth Print, Qum, Dār al-Fikr Publications, 1352 Hijrī Shamshi, p. 74-82. These numbers have been obtained through narrations from the book Muntakhab al-’Athar; with further research, the number of traditions will naturally be more.  Muḥammad ibn Jarīr ibn Rustam Ṭabarī, Dalā’il al-’Imāmah, al-ṭab’at al-thālithah, Qum, Manshūrāt al-Rāḍī, p. 240.  Refer to: Mahdī Faqīh Īmānī, Mahdī Muntaẓar in the Nahj al-Balāghah, Iṣfihān, The Amīr al-Mu’minīn (a) Public Library.  Ibn Abī al-Ḥadīd, Sharḥ Nahj al-Balāghah, Cairo, Dār Aḥyā’ al-Kutub al-’Arabīya, 1960 A.D., vol. 19, p. 29, Wisdom 205.  Surah Qaṣaṣ, Verse 6 (وَنُرِيدُ أَن نَّمُنَّ عَلَى الَّذِينَ اسْتُضْعِفُوا فِي الْأَرْضِ وَنَجْعَلَهُمْ أَئِمَّةً وَنَجْعَلَهُمُ الْوَارِثِينَ)  Ṣadūq, Kamālidīn, Qum, Mu’assasat al-Nashr al-Islāmī (al-Tabi’ah) Lijamā’at al-Mudarisīn, 1405 Hijrī Qamarī, vol. 1, p. 318.  Ṣadūq, Ibid, vol. 2, p. 377; Majlisī, Biḥār al-’Anwār, Tehran, Maktabah al-Islāmīya, 1393 Hijrī Qamarī, vol. 51, p. 156.  Taken from the text: Sīrah Pīshvayān, Mahdī Pīshvāī, Mu’asasah Imam Sadiq (‘a), Qum, 1390 Hijrī Shamsī, 23rd Edition, p. 692.