Reference for Bava Batra 252:3

מר זוטרא מדרישבא פלג בצנא דפלפלי בהדי אחין בשוה אתא לקמיה דרב אשי אמר ליה הואיל ויתרתה במקצת ויתרתה בכל הנכסים כולן:

This shows that the firstborn is not regarded as the [legal] possessor of his share<span class="x" onmousemove="('comment',' Lit., 'he has not' ');">⁷</span> before distribution [had taken place]. And the law is that the firstborn is the possessor of his share<span class="x" onmousemove="('comment',' Lit., 'he has'. ');">⁸</span> [even] before distribution [of the estate had taken place].

Jastrow