Ambrose of Milan

According to the letter of the law, a woman is compelled to marry, however unwilling, in order that a brother may raise up seed to his brother who is dead. The letter therefore killeth, but the Spirit is the master of charity.

Catena Aurea by Aquinas ↗

Bede

And they asked him, saying: Teacher, Moses wrote to us, if anyone's brother dies having a wife, and he is without children, that his brother should take his wife, and raise up seed to his brother. See the difference between the letter and the spirit. According to the letter, one is compelled to marry in life, so that the brother may raise up seed for the deceased; the spirit, however, is the measure of chastity.

On the Gospel of Luke ↗

Bonaventure

Second, regarding the inquiry of the Sadducees alleging the precept, it is added: And they asked him, saying: Master, Moses wrote for us, that is, in authentic and just scripture, according to that of Ecclesiasticus twenty-four: "Moses commanded a law in the precepts of justices"; whence Deuteronomy thirty-one: "After Moses had written the words of this Law in a volume and completed it, he commanded the Levites who carried the ark of the covenant, saying: Take this book and place it at the side of the ark," etc. …

Commentary on Luke, Chapter 20 ↗