Musnad Ahmad 1153:1785

'Abd al-Malik ibn 'Amr narrated to us, he said: 'Ali—meaning Ibn al-Mubarak—narrated to us, from Yahya, from Abu Salamah, that Umm Bakr informed him, from 'Aishah, that the Prophet, peace and blessings of Allah be upon him, said regarding a woman who sees what disturbs her after purity: "It is only a vein," or he said: "Veins."