Reference for Chullin 281:20

א"ל ר' אבא בריה דרב יוסף בר רבא לרב כהנא

And [it is established law that] a positive precept<span class="x" onmousemove="('comment',' For the fulfilment of which the bird is required, v. n. 3.');"><sup>14</sup></span>