Chasidut sur Le Cantique des Cantiques 1:9

If you pray while holding another thought in mind, it is as if - God forbid! - the klippot, the rough husks of reality are riding along on your words. Because Thought rides on Speech, and this is the meaning of (Song of Songs 1:9) "To a horse in Pharaoh's chariots -" speech is likened to horses, and when Pharaoh (that is, the foreign thought) rides upon them, then "I have likened you, my beloved." It would be far better to be silent. Yet what comes from the heart enters the heart, to the supernal heart by means of breath, as is well known.