Yevamot 90B

אמר ליה, בעאי לאותובך: ערל, הזאה.

Rav Ḥisda said to Rabba: I wanted to raise a difficulty against you from the halakha of an uncircumcised man. The rabbis decreed that one who converts on the eve of Passover may not partake of the Paschal lamb, due to his ritual impurity. According to Beit Hillel, one who separates from the foreskin by being circumcised is ritually impure like one who separates from the grave (Pesaḥim 92a). This is the halakha despite the fact that by Torah law he is obligated to bring the offering. Rav Ḥisda continued: And I also thought of asking from the case of sprinkling the waters of a purification offering for one who became ritually impure through contact with a corpse, as the rabbis rendered it prohibited for one who is impure to receive the sprinkling on the eve of Passover that occurred on Shabbat, although this prevents him from partaking of the Paschal lamb.

ואזמל, סדין בציצית.

And I was likewise going to raise a question from the case of a circumcision knife, which the rabbis decreed may not be carried on Shabbat, despite the fact that this entails the neglect of a Torah mitzva. And I also wanted to raise a question from the case of a linen cloak, on which the rabbis did not allow one to place tzitzit made of wool. This is a decree that was issued lest he do the same with a garment worn only at night, which is exempt from fringes, and therefore this would be a mixture of wool and linen that is forbidden, although this means that he is unable to fulfill the mitzva of tzitzit.