Reference for Chullin 227:32

אמרת ק"ו

From this I know [that the kid is forbidd in its mother's milk, but where do I know [that it is forbidden] in the milk of its 'older sister'!<span class="x" onmousemove="('comment',' I.e., cows, in contradistinction from 'the younger sister' i.e., sheep. This is the explanation which Rashi says he received from his teachers, but after criticizing it Rashi expresses his preference for the interpretation of R. Joseph Bonfils, according to which 'older sister' and 'younger sister' are both goats, the former, however, being a goat of last year's breeding which had already been counted with other goats for the purposes of tithing, the latter being one which has not been counted with others for tithing.');">¹⁴</span>

Jastrow