Musnad Ahmad 1153:415

'Abd al-Samad narrated to us, he said: My father narrated to me, Husayn narrated to us, from Yahya ibn Abi Kathir, from Abu Salamah, he said: And he informed me that Umm Bakr informed him, that 'Aishah said: The Messenger of Allah, may Allah bless him and grant him peace, said regarding the woman who sees what causes her doubt after the period of purity: "It is only a vein, or he said: veins."