SEPHER YETZIRAH OR THE BOOK OF CREATION : Understanding the Gra Tree and Kabbalah

The full Sefer Yetzirah (Hebrew, "Book of Formation," or "Book of Creation) is the title of the earliest extant book in Jewish esotericism. "Yetzirah" is more literally translated as "Formation"; the word "Briah" is used for "Creation". A cryptic story in the Babylonian Talmud states that "On the eve of every Shabbat, Judah ha-Nasi's pupils, Rab Hanina and Rab Hoshaiah, who devoted themselves especially to cosmogony, used to create a three-year-old calf by means of the Sefer Yetzirah, and ate it on the Sabbath.
Mystics also assert that Abraham used the same method to conjur the calf prepared for the three angels who foretold Sarah's pregnancy in the Biblical account at Genesis 18:7. All the miraculous creations attributed to other rabbis of the Talmudic era are ascribed by rabbinic commentators to the use of the same book.