Reference for Berakhot 21:4

ההוא מבעי להו לכדתניא בשבתך בביתך פרט לעוסק במצוה ובלכתך בדרך פרט לחתן מכאן אמרו הכונס את הבתולה פטור ואת האלמנה חייב

This is required in accordance with the teaching : " When thou sittest in thy house" excludes one occupied with a religious duty; "and when thou walkest by the way" excludes a bridegroom. Hence they say : He who takes a virgin to his house as wife is exempt [from the obligation of reading the Shema' on the night of his marriage] ; but if he marry a widow, he has that obligation.

Jastrow