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Seeing that Avraham served the angels mustard on Pesach (Bava Metzia 86b),

ולמה לי תלתא תסגי בחד אמר רב חנן בר רבא כדי להאכילן שלש לשונות בחרדל אמר רבי תנחום בר חנילאי לעולם אל ישנה אדם מן המנהג שהרי משה עלה למרום ולא אכל לחם מלאכי השרת ירדו למטה ואכלו לחם ואכלו סלקא דעתך אלא אימא נראו כמי שאכלו ושתו

The Gemara asks: And why do I need three calves? One calf should be sufficient for three guests. Rav Ḥanan bar Rava said: Abraham prepared three calves in order to feed the guests three tongues with mustard particular delicacy.

and the Rema says not to have mustard on Pesach (O.C. 464:1)

הגה והמנהג שלא לאכול חרדל כלל בפסח אפי' נתערב קודם פסח דהוי כמיני קטניות שנוהגין בו איסור:‏

Why would Avraham have served mustard if he kept any precautionary measure to avoid transgression:

וישמר משמרתי. גזרות להרחקה על האזהרות שבתורה, כגון שניות לעריות ושבות לשבת (יבמות כ"א):‏

וישמר משמרתי AND KEPT MY CHARGE — This refers to precautionary measures which are intended to make us avoid the infringement of Biblical prohibitions: such are the Rabbinical inhibition of marriage between relatives in the second degree and the Rabbinical regulations regarding not doing certain acts on the Sabbath.

marked as duplicate by Danny Schoemann, Joel K, DonielF, mbloch, Gershon Gold Oct 23 '18 at 19:37

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  • 3
    Kitniyos is only a minhag... – Ploni Oct 21 '18 at 3:06
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    It means Avraham was a Sefardi. – David Kenner Oct 21 '18 at 3:06
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    @DavidKenner Clearly. Sounds like the start to a good Purim Torah... – ezra Oct 21 '18 at 4:10
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    Your source only says he kept gezeiros derabbanan, not minhagim. – Ploni Oct 21 '18 at 4:12
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    @DoubleAA Can we get this closed as a dupe of judaism.stackexchange.com/q/4078/9682? – DonielF Oct 21 '18 at 10:08