הלכה על סנהדרין 42:6

It is from the roots of the commandment [that it is] because the king is under his own power - no man will pain him over his actions nor rebuke him, and with the staff of his mouth he can plague his land and with the breath of his lips he can kill anyone he wants from his entire people. Therefore he truly needs great guarding, and a proper reminder standing in front of him [so that] he will always look upon it in order to conquer his impulse and turn his heart towards his Creator. And this is [the meaning of] that which they, may their memory be blessed, said (Sanhedrin 21b), "When he goes out to war, the Torah scroll is with him; sitting in judgement, it is with him; adjourns to eat, it is in front of him." The general rule of the matter is that it not move from in front of his eyes except when he needs to [relieve himself] or enters the bathhouse.