Two parts of one Torah
The Written Torah sets the principle; the Oral Torah details how it is carried out. The verse says "bind them as a sign upon your hand", but only the tradition teaches that this means tefillin, what is written in them and how they are worn.
משה קבל תורה מסיני, ומסרה ליהושע, ויהושע לזקנים, וזקנים לנביאים, ונביאים מסרוה לאנשי כנסת הגדולה
Moses received the Torah from Sinai and passed it to Joshua, Joshua to the elders, the elders to the prophets, and the prophets passed it to the Men of the Great Assembly.