Reference for Zevachim 74:5
ותנן אמר רבי יהושע שמעתי שתמורת פסח קריבה ותמורת פסח אינה קריבה ואין לי לפרש
BUT IN THE CASE OF A SIN-OFFERING TWO APPLICATIONS [ARE INDISPENSABLE]. R. Huna said, What is Beth Shammai's reason? - The plural form karnoth [horns] is written three times, denoting six [applications], [thus intimating that] four are prescribed while two [at least] are essential. But Beth Hillel [argue]: [The written forms are] karnath [singular] twice, and karnoth [plural] once, which denotes four, implying that three [applications] are prescribed, while [only] one is essential. Yet say, that all are [only] prescribed? We find no atonement without rite. Alternatively, this is Beth Hillel's reason: Both mikra [the version as read] and masoreth [the version as traditionally written] are effective: the mikra is effective in adding one [application], while the masoreth is effective in subtracting one. <br>