What is the service of the heart? It is prayer (Ta’anit 2a ).
A person should devote one’s full concentration to the words of one’s prayers, so that it should not be said, “They honor Me with their lips, but their heart is far from Me” (Isaiah 29: 13)
Rebbe Nachman, Likutei Moharan I: 49: 2
By Yaakov Shmuel