Salaam. Can't see TTH's comment, so I'll answer based on my knowledge: The oldest known sufi is Khidhr AS, the immortal man who was ancient in the time of Musa AS, and is mentioned in Surah-e-Waqiyah as the mysterious man of Allah. He has to have personally known Imam Ali (AS), as evidenced by his beautifully eloquent prayer which Imam Ali (AS) has propagated through his companion Kumayl ibn Ziyad, known as Dua-e-Kumayl al-islam.org). Imam Ali (AS) is the next known sufi, and it is through him that all sufi schools of learning (except one, known historically) derive their lineage. Idries Shah spoke of this extensively in his books "Knowing How to Know" and "Learning How to Learn." -- I believe an argument may be made that our beloved Rasool (SAAWW) was also a sufi, as he followed the esoteric path, meditated in the cave of Hirah regularly every year in the month of Ramadan until he achieved Aarifah of Allah (the goal of any true sufi) and became the medium through which The Message of Allah (Islam) was to be conveyed to all humanity and jinn folk. (He was known to spend the entire month of Ramadan in such meditation, annually after his marriage to Ummul Momineen Khadijah al-Kubra, and a few times before, I believe. It is known that Ali AS accompanied him more than once as a child, before Revalation, on his extended stay at the cave.) Our Rasool (S)'s achievement of a direct connection with Allah had to have preceeded his receiving Relevation, possibly qualifying him as the "first sufi".. but Ali (AS) is generally acknowledged to be Islam's first sufi, from very early in his life. I hope this answers your question.

The first sofism is The prophet muhammed, peace be upon him..