Reference for Zevachim 75:4

ותנא מייתי לה מהכא (ויקרא ד, כו) וכפר וכפר וכפר מפני הדין

three times, on account of the analogy [which might otherwise be drawn], [teaching]: 'and he shall make atonement' even though he sprinkled [the blood] only three times; 'and he shall make atonement' even though he sprinkled [it] only twice; 'and he shall make atonement' even though he sprinkled it but once.

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