Musnad Ahmad 1153:2369

Hassan ibn Musa and Husayn ibn Muhammad narrated to us, they said: Shayban narrated to us, from Yahya, from Abu Salamah, from the mother of Abu Bakr, that she informed him that Aisha said regarding a woman who sees something of blood that makes her suspicious after the period of purity: "It is only a vein or veins."