Reference for Yevamot 39:14
תניא רבי יהודה מחליף איסור מצוה אלמנה לכהן גדול גרושה וחלוצה לכהן הדיוט ואמאי קרי ליה איסור מצוה דכתי' (ויקרא כז, לד) אלה המצות
PROHIBITED BY VIRTUE OF A COMMANDMENT', [REFERS TO] THE SECONDARY DEGREES. Why are these designated, PROHIBITED BY VIRTUE OF A COMMANDMENT'? — Abaye replied: Because it is a commandment to obey the rulings of the Sages. PROHIBITED BY VIRTUE OF HOLINESS' … A WIDOW TO A HIGH PRIEST; A DIVORCED WOMAN, OR ONE WHO HAD PERFORMED THE <i>HALIZAH</i>, TO A COMMON PRIEST. Why are these designated 'PROHIBITED BY VIRTUE OF HOLINESS'? — Because It is written in the Scriptures, They shall be holy onto their God.<span class="x" onmousemove="('comment',' Lev. XXI, 6. ');"><sup>34</sup></span> It was taught: R. Judah reverses the order: prohibited by virtue of a commandment [refers to the following prohibited categories:] a widow to a high priest; a divorced woman or one that had performed <i>halizah</i>, to a common priest. And why are these designated, prohibited by virtue of a commandment? — Because it is written in the Scriptures, These are the commandments.<span class="x" onmousemove="('comment',' Lev. XXVII, 34 which refers to all the priestly commandments laid down in that book. ');"><sup>35</sup></span>