Tertullian, de Coronâ, 3:  At every forward step and movement, at every going in and out, when we put on our clothes and shoes, when we bathe, when we sit at table, when we light the lamps, on couch, on seat, in all the ordinary actions of daily life, we trace upon the forehead the Sign.  If for these, and other such rules, you insist upon having positive Scripture injunction, you will find none.  Tradition will be held forth to you as the originator of them, custom as their strengthener, and faith as their observer.