The Mahdi of Islam will have long occultation (Ghaybat) and after its completion, he will "re-appear" (Zohoor).

This narration is reported through 91 traditions by both Shia and Sunni traditionalists through reports from Jabir Ibne Abdullah Ansari, Ali Ibne Abi Talib (as), Abu Abdillah, Abu Jafar, Ibne Abbas, Imam Reza (as) and others.

For the benefit of my readers, a few references for this tradition are given below:
1.   Kamaluddin, volume 1, chapter 25, page 287, tradition 5
2.   Ibid, volume 2, chapter 38, page 394, tradition 4
3.   Ibid, volume 1, chapter 25, page 286, tradition 3
4.   Ibid, volume 2, chapter 35, page 372, tradition 2
5.   Ibid, volume 1, chapter 6, page 145, tradition 12
6.   Ibid, volume 1, page 51
7.   Fara'edus Simtayn, volume 2, page 335, tradition 574
8.   Isbatul Huda volume 3, page 461, chapter 32, part 5, tradition 587
9.   Ibid, volume 3, page 460, chapter 32, part 5, tradition 503
10. Ibid, volume 2, page 574, chapter 32, part 47, tradition 710
11. Ibid, volume 1, page 550, chapter 9, part 18, tradition 378
12. Beharul Anwar, volume 51, page 68, chapter 1, tradition 257
13. Ibid, volume 52, page 90, chapter 20, tradition 1
14. Ibid, volume 13, page 36, chapter 2, tradition 17
15. Ibid, volume 49, page 237, chapter 17, tradition 6
16. Ibid, volume 51, page 156, chapter 8, tradition 4
17. Ibid, volume 53, page 65, chapter 29, tradition 56
18. Ibid, volume 36, page 309, chapter 41, tradition 148
19. Ibid, volume 51, page 71-72, chapter 1, tradition 13, 16
20. Ghayatul Meraam, chapter 141, page 695, tradition 30
21. Yanabiul Mawaddah, page 488, chapter 94
22. Muntakhabul Asar, chapter 25, part 2, page 249, tradition 8

In addition to the above, Mirza Ali Mohammed Shirazi himself acknowledged the Ghaybat of the Mahdi of Islam in his book Sahifae Adaliyah.

The word used in tradition indicates that the Mahdi will "re-appear" after his occultation, not that "he will be born". This refutes the Bahai belief that the Bab was the return of the Mahdi.

Mirza Ali Mohammed Shirazi, The Bab too acknowledged the Ghaybat of the Twelfth Imam in his work - Tafseere' Surah Kauthar.

Bahais argue that the time spent by the Bab in captivity was the equivalent of Ghaybat of the Bab. However, they are mistaken for two reasons. This implies that every criminal who ever spent time in jail is eligible for the position of Ghaybat. Secondly, the definition of Ghaybat found in traditions is "Yarawnahu wala Ya'refunahu" - people will see him, but not recognise that he is the Twelfth Imam. The example of this concept is found in the events of Hazrat Yusuf (as) whose brothers met him while he was the King of Egypt. They saw him but failed to recognise that he was the same Yusuf who they had tried to kill in his childhood.