BT Zev 5b(17) :אשם מאי קריב מותרי חטאת נמי מיקרב קריב מותרה‏ : in what (manner) is a guilt-offering offered? (As) its remainder; and certainly the remainder of a sin-offering is also offered
See the entire entry for mōṯār, mōṯārā n.m. surplus