Musar zu Jeschijahu 62:6

We must therefore refer to the Zohar who explains on Genesis 11,31 (Sullam edition Lech Lecha page 11) in which the Torah reports Terach, Abraham, Lot and family as having left Ur Kasdim with the intention of moving to the land of Canaan, that the intention to move to Canaan sufficed for G–d to offer Abraham assistance. Up to that point Abraham's intellectual achievements in the field of philosophy had been unassisted. From this point on he received Divine guidance. The crucial factor in bringing about G–d's manifest involvement was that Abraham had made the first move. We learn from here that deeds in our world act as the catalyst that evokes heavenly responses. As soon as Abraham had demonstrated his intention to move to Canaan, G–d told him לך לך. We can apply the same to the ultimate redemption, i.e. until "black light" wakes up, "white light" will not be able to merge with it and suffuse it, i.e. restore the state of the world to what it had been prior to Adam's sin.. This is the message in Isaiah 62,6 that המזכירים את ה' אל דמי לכם, "when (you -the watchmen-) mention G–d, do not practice silence." The prophet cautions the watchmen that silent contemplation is not enough. Whenever man makes the first move towards Him G–d can be depended on to respond and lead man on to greater things. Thus far the Zohar.

I have found in the book Ba-assarah ma-amorot chapter 3, an article that explains the source of this legislation. The author writes as follows, quoting Isaiah 62,6: "Upon your walls O Jerusalem, I have set watchmen." These are four watchmen corresponding to the four-lettered name of G–d, their דינים are three, since the watchmen of the "Upper World" never abandon Jerusalem, and concerning them the prophet says "all day long and all night long" (ibid.). They are never without supervision.