הלכה על יבמות 41:16

From the laws of the commandment is that which they, may their memory be blessed, forbade (Yevamot 21a) the daughter of the son of her son as a fence for this - and it is included in the secondary [prohibitions]. As everything the prohibition of which is from the words of the [Rabbis] is called a secondary prohibition. And so [too,] did they forbid only the mother of the mother of the father of his wife.