"The sanctity of tefillin is very great. As long as the tefillin are on the head and on the arm of a man, he is modest and God-fearing and will not be attracted by hilarity or idle talk, and will have no evil thoughts, but will devote all his thoughts to truth and righteousness; ........ Although the tradition enjoins the laying of tefillin the whole day, it is especially commendable to lay them during prayer. The sages say that one who reads the Shema without tefillin is as if he testified falsely against himself. He who does not lay tefillin transgresses eight commandments; for in each of the four Biblical passages there is a commandment to lay tefillin on the head and on the arm. But he who is accustomed to lay tefillin will live long, as it is written, 'When the Lord is upon them they will live'".

                        - Miamonides, Mishneh Torah