"And he said unto the LORD: Turn not Thou unto their offering" (ibid., Numbers 16:15) — that is, do not receive them back in repentance. Scripture ought to have said, "Turn not unto their service." What, then, is the meaning of "unto their offering"? Thus did Moses say before the Holy One, blessed be He: Master of the World, I know that these men have a share in that offering which has been presented, as it is stated, "Besides the continual burnt offering, and the meal offering thereof, and the drink offering thereof" (Numbers 28:31), and it belonged to all Israel and was offered up. Since these men have separated themselves from among Your children, do not look upon their portion; let the fire leave it and not consume it.
Why Moses Prayed God Reject Korach's Meal Offering
Midrash Tanchuma Buber, Korach 18:1
ויאמר אל ה' אל תפן אל מנחתם (שם). אל תקבלם בתשובה, היה צריך למקרא לומר אל תפן אל עבודתם, מהו אל מנחתם, כך אמר לפני הקב"ה, רבש"ע יודע אני שיש לאלו חלק באותה מנחה שהקריב, [שנאמר] מלבד עולת התמיד ומנחתה ונסכה (במדבר כח לא), והיתה של כל ישראל קריבה, הואיל ופירשו אילו מבניך, אל תסתכל בחלקם, תניחנו האש ולא תאכלנו.