Reference for Temurah 49:19

ת"ל

Said R'Amram to R'Shesheth: If one says of a firstling at the moment that the greater part of it was emerging from the womb:<span class="x" onmousemove="('comment',' While still inside the animal, at the time when the holiness of a firstling takes effect.');">¹³</span>

Jastrow