From Al-Ghazali on Disciplining the Soul and on Breaking the Two Desires : Books Xxii and Xxiii of the Revival of the Religious Sciences as recorded on

Abu Hamid al-Ghazali (1058-1111) theologian, logician, jurist and mystic, was born and died at the town of Tus in Central Asia, but spent much of his life lecturing at Baghdad or leading the life of a wandering dervish. Because of his success in revealing the compatibility of the outward forms of religion with the inner experiences of Sufi tradition, he is commonly regarded as the 'Renewer' of the fifth Muslim century, and the most influential thinker of medieval Islam.

I have heard it said by some that Al-Ghazali is the most influential Muslim after Muhammad.