John Gill Commentary Luke 20:28

"and they asked him, saying, Teacher, Moses wrote unto us, that if a man`s brother die, having a wife, and he be childless, his brother should take the wife, and raise up seed unto his brother." — Luke 20:28 (ASV)

Saying, master, Moses wrote unto us
In (Deuteronomy 25:5) where the substance of what follows is contained, though not in express words:

if any man's brother die, having a wife, and he die without
children, that his brother should take his wife, and raise up
seed unto his brother ;
the meaning of which is, that if a man died without issue, and left a wife behind him, his next brother, if unmarried, was to marry his wife, and the first child born of her, was to be reckoned the deceased's, and to inherit his estate; (See Gill on Matthew 22:24).