Abu Sa'id ibn Abi'l-Khayr (d. 440/1049), was an early Sufi shaykh who at different stages of his life was an ascetic, an antinomian ecstatic, and a spiritual guide. He received a Sufi transmission from Abu al-Fadl al-Hasan (or ibn al-Hasan) al-Sarakhsi, whom Abu Sa'id called his "pir" (a Persian word refering to a spiritual guide and often equivalent to "shaykh"). After the death of Abu al-Fadl, Abu Sa'id looked to Abu 'Abbas al-Qassab (the butcher), whom Abu Sa'id called "shaykh," for spiritual guidance. The hagiography Asrar al-tawhid is one of the two major sources for what we know of his life and teachings. It has been translated as The Secrets of God's Mystical Oneness.
Source : http://www.uga.edu/islam/sufismearly.html