Zevachim 101A

וחכמים עשו חיזוק לדבריהם יותר משל תורה.

And the same holds for the night after the day of burial, even though the acute mourning of that day itself is by rabbinic law, because the rabbis reinforced their pronouncements with greater severity than Torah law.

תנו רבנן: ״כי כן צויתי״, ״כאשר צויתי״, ״כאשר צוה ה׳״; ״כי כן צויתי״⁠ – באנינות יאכלוה, ״כאשר צויתי״⁠ – בשעת מעשה אמר להן, ״כאשר צוה ה׳״⁠ – לא מאליי אני אומר.

§ A baraita states: On the 8th day of the inauguration of the Tabernacle, on which two of Aaron’s sons died, Moses spoke to Aaron and his sons using 3 different forms of the word command: “For so I am commanded [tzuveiti]” (Leviticus 10:13), “as I commanded [tziveiti]” (Leviticus 10:18), and “as YHWH has commanded [tziva]” (Leviticus 10:15). Moses said to Aaron: “And you should eat it…for so I am commanded,” to teach that Aaron and his remaining sons should partake of the offerings even in acute mourning. The statement: “You should certainly have eaten it…as I commanded,” Moses said to them at the time of the incident, when Aaron and his sons burned the sin offering for Rosh Chodesh. Moses said: “As YHWH has commanded,” to emphasize that it is not of my own initiative that I say this, but it is from the word of God.

ורמינהי: מפני אנינות נשרפה, לכך נאמר: ״כאלה״!

And the rabbis raise a contradiction from another baraita: The sin offering was burned due to the acute mourning of Aaron and his sons, since they felt they could not partake of it. Therefore, it is stated in Aaron’s explanation: “There have befallen me such things as these; and if I had consumed the sin offering today, would it have been good in the eyes of YHWH?” (Leviticus 10:19). Moses conceded to Aaron that he was correct (see Leviticus 10:20), indicating that it was not permitted for Aaron to partake of the sin offering in acute mourning.