Reference for Bava Batra 248:7

אמר רב יהודה אמר שמואל אין בכור נוטל פי שנים במלוה למאן אילימא לרבנן השתא שבחא דאיתיה ברשותיה אמרי רבנן לא שקיל מלוה מבעיא

Of all that he hath,<span class="x" onmousemove="('comment',' Deut. XXI, 17. ');">¹¹</span> excludes the appreciation [of an estate] which the heirs have produced after the death of their father; but [in] the [natural] appreciation of the estate [that accrued] after the death of their father he [does] take [a double portion]. And who is [the author of this statement]? — It is Rabbi.

Jastrow