Yosef Karo, Translation:Shulchan Aruch/Even ha-Ezer…
“ Paragraph 3- When the purity of a families lineage has been disputed, which means two people testify that there was i a mamzer or a challal intermingled with them or that they have slaves in their lineage, behold this is a doubtful family. If it is a family of priests no one may marry a women from them until they will check about her up through four generations, totaling eight people: her mother, her maternal grandmother, the mother of her maternal grandfather, the maternal grandmother of her maternal grandfather. ”
