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is not because of the willingness of Imam Mahdi itself but because of God's sure destiny.
Imam Mahdi himself did not realize that he was the Mahdi Imam but after Allah SWT put it in one night, as said in a hadith:
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From Ali bin Abi Thalib, that the Prophet sallallaahu 'alaihi wa sallam said,
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المهدى منا أهل البيت يصلحه الله فى ليلة
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Al-Mahdi belongs to our group, a member of the temple, God fixed it overnight. (HR Ahmad 655, Ibn Majah 4223, cleared Ahmad Syakir and assess Hasan by al-Albani). _
Source @ hendri.jef