Radak

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.