Sahl al-Tustari was born in the fortress town of Tustar (Arabic) or Shushtar (Persian) during the golden age of the Abbasid Caliphate, in Khūzestān Province in what is now southwestern Iran.
[1] From an early age he led an ascetic life with frequent fasting and study of the Qur'an and Hadith, the oral traditions, of the Prophet Muhammad.
This eventually culminated in a direct and intimate rapport with God with whom he considered himself a special friend and one of the spiritual elect.
Another key idea that he unravelled was the meaning of the Prophet Muhammad's saying "I am He and He is I, save that I am I, and He is He", explaining it "as a mystery of union and realization at the center of the Saint's personality, called the sirr ('the secret'), or the heart, where existence joins Being.
"[7] Tustari also "was the first to put" the Sufi exercise of remembrance of God, Dhikr, "on a firm theoretical basis.