Temurah 21B

ואתי מעשר ויליף מבכור. ומי ילפינן (קדש) [בקדשים] מהדדי? והאמר רבי יוחנן: כל התורה כולה למדין למד מן הלמד, חוץ מן הקדשים שאין למדין למד מן הלמד!

And then, as R' Yishmael holds that a firstborn definitely may not be eaten in the present, he says that second tithe comes and its status is derived from the juxtaposition to the firstborn. The Talmud asks: And can we derive the halakhot of sacrificial food from one another? But doesn’t R' Yoḥanan say: In the entire Torah, we may derive that which is derived from a halakha that was itself derived from another source, except for the case of sacrificial animals, where we do not derive that which is derived from that which was already derived from another source.

מעשר חולין הוא. הניחא למאן דאמר בתר למד אזלינן, אלא למאן דאמר בתר מלמד אזלינן, מאי איכא למימר? בשר ודם חדא מילתא היא.

The Talmud answers: Second tithe is considered non-sacred. Therefore, this derivation does not pertain to consecrated items. The Talmud asks: This works out well according to the one who said that we follow the halakha that is derived. If it is a consecrated item, it may not be derived from another halakha that was derived from another source, but if it is non-sacred, such as second tithe, it may be derived in this manner. But according to the one who said that we follow the halakha that teaches, what is there to say? In this case the halakha that teaches, i.e., the source of the derivation, is the meat of the firstborn, which is a consecrated item that is derived itself from the status of the blood of the firstborn. The Talmud answers: The status of the meat and the blood of a firstborn offering are one matter. Therefore, the meat is not considered as derived from another halakha.