Tosefta zu Dewarim 17:18

And he writes for himself a Sefer Torah to learn without ulterior motivation, that it should not be in the house of his ancestors, but in his house, as it says "and he writes" (Deut. 17:18), meaning, that it is written in his name and a regular person has no permission to read from it, as it is written "and he will read" (Deut. 17:19) he, and not a regular person. And we leave it with a Beit Din of Kohanim, with a Beit Din of Levvim and with a Beit Din of Israelites who are able to marry into the Kehunah. 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 bathing house 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."