מדרש על חולין 273:19

This tells me only of the bechor of a bullock (as being forbidden) for work, and the bechor of a sheep, for shearing. Whence do I derive (the same for) the reverse? It is derived a fortiori: If a blemished bullock, which is not equivalent to an unblemished bullock to be sacrificed on the altar, is equivalent to it (to be forbidden) for work, then an unblemished sheep, which is equivalent to an unblemished bullock to be sacrificed on the altar, how much more so should it be equivalent to it (to be forbidden) for work! And the same for shearing, viz.: If a blemished sheep, which is not equivalent to an unblemished sheep to be sacrificed on the altar, is equivalent to it (to be forbidden) for shearing, then an unblemished bullock, which is equivalent to an unblemished sheep to be sacrificed on the altar, how much more so should it be equivalent to it (to be forbidden) for shearing! But, still, we have derived the rule for unblemished (animals). Whence do we derive the same for blemished ones? It follows a fortiori, viz.: If an unblemished sheep, which is not equivalent to a blemished sheep, to be eaten outside of Jerusalem, is equivalent to it (to be forbidden) for shearing, then a blemished bullock, which is equivalent to a blemished sheep to be eaten outside of Jerusalem, how much more so should it be equivalent to it (to be forbidden) for shearing!

And how many sheep must there be for (the obligation of) the first of the shearing? Beth Shammai say: Two ewes, viz. (Isaiah 7:21) "And it shall be on that day that each man will raise a heifer and two sheep.