Reference for Keritot 29:23
לא שמענו אבל שמענו
with his five wives, while they were menstruant, whereby only one prohibition has been transgressed, he is liable to a sacrifice for each act of transgressing the law concerning menstruant women; and it seems to us that the case [you state] may be derived therefrom by an a fortiori conclusion [thus]: If one is liable to separate offerings in the case of intercourse together with his five menstruant wives, whereby only one prohibition has been transgressed, how much more should one be liable to separate offerings in the case of the sister who is at the same time his father's sister and his mother's sister, whereby three different prohibitions have been transgressed! But [against this conclusion] one may object: the case of the five menstruant women [is rightly more stringent] because several persons [are involved]?