It is narrated from Sayyiduna Anas bin Malik (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) ascertained that the womb of Sayyidah Safiyyah (may Allah be pleased with her) was clear by waiting for one menstrual period.
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 20554