Sepher Yetzirah:

“According to Eliphas Levi, the three greatest books of Qabbalism are the Sepher Yetzirah, The Book of Formation; the Sepher ha Zohar, The Book of Splendor; and the Apocalypse, The Book of Revelation. The dates of the writing of these books are by no means thoroughly established. Qabbalists declare that the Sepher Yetzirah was written by Abraham. Although it is by far the oldest of the Qabbalistic books, it was probably from the pen of the Rabbi Akiba, A.D. 120. ”

– Manly P. Hall, The Secret Teachings of All Ages