The claim
Al-Ghazali's turn from teaching to Sufism is documented in his own account.
The evidence
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The chain of transmitters — passage/muwatta-1-037
A Muwatta hadith with isnad.
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chronicle — work/al-munqidh-min-al-dalal
A paragraph of al-Munqidh where al-Ghazali tells his crisis and turn, faithfully rephrased.
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object — work/al-mughni
Al-Mughni in Hanbali fiqh — the legal world al-Ghazali inhabited.
The historians' note
The Deliverer from Error describes his turn in his own words; researchers read it as self-testimony, not neutral witness.