Midrash for Sanhedrin 37:3

תניא נמי הכי אם קדמו ובעלו ביאה ראשונה קנו ואסור לקיימן בביאה שניה:

It has been taught likewise: [Where the widow is forbidden in marriage to the brother-in-law by a negative or positive command] and he has connubial relations at all with her, he acquires [her in marriage] but may not retain her for further cohabitation.<span class="x" onmousemove="('comment',' V. Yeb. 20b. This proves that a second act of connubial relationship is forbidden. ');">⁵</span> IF A DEATH HAPPENS IN HIS FAMILY. Our Rabbis taught: Neither shall he go out of the Sanctuary:<span class="x" onmousemove="('comment',' Lev. XXI, 12. ');">⁶</span>

Sifrei Devarim