Rashi on Exodus 24:6/רש"י על שמות כ״ד:ו׳

1ויקח משה חצי הדם AND MOSES TOOK THE HALF OF THE BLOOD — Who divided it into halves? An angel came down and divided it (Leviticus Rabbah 6:5).

2באגנת IN BASONS — There were two basons, one for holding the half of the blood of the burnt offering and the other for holding the half of the blood of the peace offerings, in order to sprinkle it (both bloods) on the people. From here have our Rabbis inferred (Keritot 9a) that our ancestors entered into the covenant with God by means of circumcision, immersion and sprinkling of blood — and although immersion is not mentioned in this paragraph it must have taken place, for no sprinkling is effective without immersion preceding it (cf. Tosafot Yevamot 46b ד"ה דאין הזאה בלא טבילה).‎