Rashi on Deuteronomy 12:21/רש"י על דברים י״ב:כ״א

1כי ירחק ממך המקום IF THE PLACE [WHICH THE LORD THY GOD HAS CHOSEN TO PUT HIS NAME THERE] BE TOO FAR FROM THEE and thou therefore art unable to come and prepare peace-offerings at any time, as thou doest now when the Tabernacle accompanies you,

2וזבחת … כאשר צויתך THEN THOU MAYEST SLAUGHTER … AS I HAVE COMMANDED THEE — This teaches us that there was already a commandment regarding the slaughtering of animals — as to how one should slaughter; it is not written in the Torah but it comprises the traditional regulations regarding the slaughter of animals that were given orally (נאמרו) to Moses on Mount Sinai (Sifrei Devarim 75:7; Chullin 28a).