Reference for Chullin 227:51

אף אני אביא אחותו קטנה שהוא בשר ואסור לבשל בחלב

and in the milk of each [the kid] may not be cooked; thus I will include 'the younger sister' too, for since it is flesh,<span class="x" onmousemove="('comment',' According to some MSS. the reading is 'it is milk and it is forbidden to cook in it' instead of 'is flesh etc.', and so it appears from Rashi too. V. Glos. of Maharam Schiff a.l.');">²³</span>

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