Rashi on Deuteronomy 11:18/רש"י על דברים י״א:י״ח

1ושמתם את דברי AND YE SHALL PLACE MY WORDS [UPON YOUR HEART] — Even after you have been banished make yourselves distinctive by means of My commands: lay Tephillin, attach Mezuzoth to your doorposts, so that these shall not be novelties to you when you return. Similarly does it state, (Jeremiah 31:21) “Set thee up distinguishing marks” (Sifrei Devarim 43:34).