Reference for Bava Batra 245:2
או כלך לדרך זו חלקו עם אחד וחלקו עם ה' מה חלקו עם אחד פי שנים בכל הנכסים אף חלקו עם ה' פי שנים בכל הנכסים
[let] his share [when he is co-heir] with one [brother] be compared with his share [when co-heir] with five [brothers]; as his share [when co-heir] with one is a double portion in all the estate<span class="x" onmousemove="('comment',' I.e., two thirds of the estate. In whatever way the division is arrived at, the double portion will, in this case, always consist of two thirds of the entire estate. ');"><sup>3</sup></span> so [is the case when he inherits] his share with five [he should also receive] a double portion in all the estate?<span class="x" onmousemove="('comment',' The firstborn should receive two thirds of the estate, and all the others together one third. ');"><sup>4</sup></span>