Kabbalah su Salmi 16:8

And learn, that the Holy One of Blessing commanded to the king "It is to remain beside him, and he is to read out of it all the days of his life, in order that he may learn to have-awe-for Ad-nai his God, to be careful concerning all the words of this Instruction and these laws, to observe them" (Deut. 17:19) and the explanation of Rashi z"l is that a sefer Torah should be with him always so he will be warned about awe, and this is so he will learn. And this is the text in the Tosefta (Tosefta Sanhedrin 4:4): "he goes out to war and it is with him, comes back and it is with him, sits in the house and it is with him, enters in the bathroom and it awaits at the door, and so too David said 'I have Ad-nai in front of me always etc' (Ps. 16:8) Rabbi Yehuda says: sefer Torah on his right hand and tefilin on his left." And it says more in the continuation: and if so, why doe it say "and he will write a second to this Torah" (Deut. 17:18)? It teaches that he writes two Torot, one with which he comes and goes and one that stays inside his house, and this that comes and goes with him does not enter with him into the bathhouse and not to the bathroom, as it is written "and it will be with him and he will read all the days of his life" - in a place where it is fit to be read, and isn't this a kal-va-chomer (a fortiori) argument, a Jewish king who is only to take care of the needs of the public has to have it and read it all the days of his life, the rest of humans all the more so.