Likutei Moharan 49:2/ליקוטי מוהר"ן מ״ט:ב׳

12. This corresponds to (Deuteronomy 11:13) “and serving Him with all your heart.” What is the service of the heart? It is prayer (Taanit 2a). For prayer is the aspect of the kingship of David, as it is written (Psalms 109:4), “but I am prayer.” And prayer, in the main, is dependent upon the heart. One should devote his entire heart to it, so that it is not in the aspect of ‘and they honor Me with their lips, but their heart is far from Me’ (Isaiah 29:13). This is because prayer in the heart is the aspect of revealing His Kingship within the Vacated Space, within the attributes, within the worlds.