Reference for Zevachim 150:2

לדבריו דר"א קאמר ואיבעית אימא הזאה לחוד ונתינה לחוד:

IF THEY WERE MIXED UP WITH A FIRSTLING OR TITHE etc. Rami b. Hama said: According to Beth Shammai, a firstling may not be given as food to menstruant women; what about its substitute? A firstling cannot be redeemed; what about its substitute? A firstling cannot be weighed by the pound; what about its substitute? - Said Raba: It was taught: A firstling and tithe, [even] when they became blemished, effect substitution. and their substitute is like themselves. <br>

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