Moshe Eliaqim Beriah on Combination of Letters in Sefer Yetzirah

"It is written in Sefer Yetzirah that the letters are called stones and the words are called houses. And the person who approaches the study of Torah and prayer, ought to build a house, which is the combinations of letters, filled by illumination and perfection and to prepare a Tabernacle for God, Blessed be He, to dwell there in those words of the prayer."

-- Moshe Eliaqim Beriah, Qohelet Moshe 8a (1875)

Rabbi Moshe Cordovero on the Necessity of the Ten Sefirot

"It is incumbent upon the sefirot to be ten, neither nine nor eleven, since the upper number is the source of the sefirot, whereas its assemblage is their boundary and number."

-- Rabbi Moshe Cordovero, Eilimah Rabbati - Ein Kol 4:16 (transl. Z. Raviv in "Decoding the Dogma within the Enigma", 2007)