Vayikra 15:24

ותהי נדתה עליו [AND IF ANY MAN LIE WITH HER AT ALL] THEN SHALL HER UNCLEANLINESS OF SEPARATION BE UPON HIM — One might think that he follows in her wake — that if he had intercourse with her e. g. on the fifth day of her separation he shall become unclean for three days only (for the fifth, sixth and seventh) like herself (i. e. because these are the remaining days during which she is unclean)! Scripture, however, states: "he shall be unclean seven days”. Therefore these words cannot refer to the duration of her uncleanness. What then is the force of ותהי נדתה עליו? They mean: her state of uncleanness as a נדה shall be upon him. How is it in the case of her? She, being an אב הטומאה, makes man unclean, and also earthen vessels by means of ‎‎היסט! So, too, he becomes an אב הטומאה (not a ולד הטומאה), and makes man unclean, and also earthen vessels by means‎ of היסט! (Niddah 33a)

וְאִ֡ם שָׁכֹב֩ יִשְׁכַּ֨ב אִ֜ישׁ אֹתָ֗הּ וּתְהִ֤י נִדָּתָהּ֙ עָלָ֔יו וְטָמֵ֖א שִׁבְעַ֣ת יָמִ֑ים וְכׇל־הַמִּשְׁכָּ֛ב אֲשֶׁר־יִשְׁכַּ֥ב עָלָ֖יו יִטְמָֽא׃ {ס}        

And if any man lie with her at all, and her menstrual flow be upon him, he shall be unclean seven days: and all the bed on which he lies shall be unclean.