Isaac the Blind, Cited by Ezra ben Solomon of Gerona in his Commentary on Canticles, in Kitve Ramban, 2: 522

Jewish Mysticism
The Pious one, our teacher, blessed be his memory said- The essence of the worship of the enlightened (maskilim) and those who meditate on His name (hoshve shemo) is “and cleave to Him” (Deut. 13-5). This is a cardinal principal in the Torah with respect to prayer and blessings, that one must harmonize one’s thought and one’s faith as if it cleaved above, to unify the name in its letters and to comprise within it the ten emanations (sefirot) like a flame bound to the coal. With his mouth he mentions it according to is appellation [Adonai], but in his heart he unites it in accordance with its structure and how it is written [YHWH].

Translated by Elliot Wolfson in Through a Speculum that Shines- Vision and Imagination in Medieval Jewish Mysticism, Princeton- Princeton University Press, 1994.