Musannaf Ibn Abi Shayba 1:1005

'Abd al-Wahhab al-Thaqafi narrated to us, from Yahya ibn Sa'id, who said: I sent [a messenger] to Rayta, the freedwoman of 'Amrah, and the messenger informed me that she said: 'Amrah used to say to the women: "If one of you inserts the *kursuf* (cotton pad) and it comes out changed, then do not pray until you see nothing [of the discharge]."