חסידות אויף תהילים 105:41

And "heart" is the aspect of "selah" (rock), that from there the speech, in the aspect of "words in rock" ([Talmud] Megillah 18). And selah (rock) is the aspect of tzur (rock, flint), as in the aspect of "He opened a rock and the waters gushed out" (Psalms 105:41), and this is the aspect of "lev" (heart), in the aspect of "the rock of my heart".