Psalms 4:5

Quake Tremble before the Holy One, blessed be He, and do not sin.

say [this] in your heart on your bed and be forever silent Bring back to your heart [the thought] that the Holy One, blessed be He, admonished [you] concerning that.

Tremble (רִגְזוּ - rigzu): Because they shamed him instead of [giving him] his glory, and spoke falsehood concerning him [v. 3], he said: Tremble (Rigzu) out of [fear of] HaShem; fear (pachadu). Similar [is the usage of the root in (Genesis 45:24)]: "Do not tremble/quarrel (al tirg'zu) on the way." [The reason for fear is] because the king takes note of the mockery directed at him. And some say (v'yesh omrim): [That rigzu means "be angry," and the verse means:] Even if you think falsehood in your hearts, let it not come out of your mouths. But this is not correct (ve-zeh eineno nakhon). For it is written (Psalm 15:2): "who speaks truth in his heart," [and (Proverbs 6:18)]: "a heart that devises wicked thoughts (machshevot aven)." The speech of the heart is the essential thing (ha-ikkar), and the tongue is not, but rather [it is merely] an interpreter (melitz) between the heart and the hearers. Rather, the interpretation is: That you should think in your hearts at night upon your beds—when you have no involvement with other people—so that you will not speak falsehood during the day. And this [leads to the final phrase]: "And be silent/still. Selah" (וְדֹמּוּ סֶלָה - Ve-dommu selah).

Tremble: – The meaning is “fear,” as “and shall tremble and be in anguish” (Deut. 2:25). Fear God if ye do not fear me.

and sin not: – in still pursuing after and rebelling against me.

Say it in your heart (i.e. think) upon your bed: – for a man upon his bed finds his heart freer from the concerns of the world and then will his intention be pure. And David addresses them: “Consider in your heart”. And so the thought of the heart is called “speaking” (as in the text) “That saith in her heart” (Is. 47:8). And he says

Be still. Selah: – His meaning is: “Be silent, and cease to rebel against me;” for “stillness” has the sense of silence of speech and voice, as “They kept still for my counsel” (Job 29:21); “stillness and a voice I heard” (ibid. 4:16). There is also (a stillness) which is (equivalent to) silence of action and movement, as “He turneth the storm to stillness” (Ps. 107:29); “my bowels boil and are not still” (Job 30:27); “and the sun stood still” (Josh. 10:13); “Be still (tarry) until we come unto you” (1 Sam. 14:9); “To the still stone” (Hab. 2:19), and (other instances) similar to these. And of this meaning is (the sentence) Be still: Selah, as though to say: “Cease you from your works.” And the great teacher of righteousness, our master Moses ben Maimon, has explained (Guide for the Perplexed, Bk. 1:50, 59) that this verse commands men of intelligence and reputation to recognise the truth and the praise of God in their hearts, so that no error of speech shall overtake them. But this is not (derived) from the sense of the Psalm.