Allah -Glory be to He- in verse 62 of the Surah Naml (27) of the Karim Qur'an has stated:

Or, Who answers 'the Moztar' [the Distressed] -when he calls upon Him-, and removes the suffering, and makes you the successors and viceroys of the earth? Is there a god with Allah? Little it is that ye heed!

Ali son of Ibrahim Qommi in his very authentic and valuable Tafseer, has narrated from Imam Mohammad Baaqer (PBUH) who has said:

Swear by Allah, it is as if I am seeing the Qaa'em [PBUH] as he is leaning on the Hajar-Ol-Aswad, and requesting his [Allah-Promised] Right from Allah; then he shall say: "O' people! Any one arguing with me in regard to Allah, I am the closest to Allah; any one arguing with me in regard to Aadam [Adam (PBUH)], I am the closest to Aadam; any one arguing with me in regard to Nooh [Noah (PBUH)], I am the closest to Nooh; any one arguing with me in regard to Ebraahim [Ibrahim (PBUH)], I am the closest to Ebraahim; any one arguing with me in regard to Moosa [Moses (PBUH)], I am the closest to Moosa; any one arguing with me in regard to 'Eesa [Jesus (PBUH)], I am the closest to 'Eesa; any one arguing with me in regard to the Rasool of Allah [Muhammad (PBUH&HP)], I am the closest to the Rasool of Allah; any one arguing with me in regard to the Book of Allah, I am the closest to the Book of Allah." Then he shall go to the 'Maqaam of Ebraahim' [standing place of the Prophet Ebraahim next to Ka'bah], and will perform two Rak'ats of Salaat and shall seek his [Allah-Promised] Right.

Then Imam Baaqer (PBUH) continued:

Swear by Allah, he [Mahdi (PBUH)] is "The Moztar" [the Distressed] who is mentioned in the Book of Allah, where it is said: "Who answers 'the Moztar' [the Distressed] -when he calls upon Him-, and removes the suffering?" … .

O' our most beautiful hope and wish, O' our beloved Mahdi (Peace be upon you and your Pure mother, Fatimah, and forefathers)! We implore Allah to hasten that wonderful moment when you will seek your Emergence next to Ka'bah. But Ah! Just thinking about the fact that you will actually feel so extremely pained and 'Distressed' that you will request your Appearance in the state of Distressedness, you the Hojjat of Allah, whose request is undoubtedly granted by Allah, Ah! Just thinking about your Distressedness at that moment, makes us want to die from sorrow, pain and shame; after all, your pain and sorrow and your extreme Distressedness, are all because of our many many shortcomings and disloyalty as the so-called "your Shi'ah", and in general due to all the disobedience, injustice, polytheism, atheism, corruption, sins, indecencies, lies, deceits, deviations, and … existing in the world. But our shameful hearts and repenting souls are overwhelmed by Allah's Glory and Grace, realizing that He has Promised that He "removes the suffering"; and this means that finally there shall come the day when you, the beloved Proof of Allah, will no longer be sad and distressed, and thus Allah will be Pleased … .

… Where is 'the Moztar' who, when calling, will be answered? …

(The Nodbah supplication, narrated from Imam Saadeq -PBUH-)