Yahya related to me from Malik from AIqama ibn Abi AIqama thathis mother, the mawla of A'isha, umm al-muminin, said, "Women used tosend little boxes to A'isha, umm al-muminin, with a piece of cottoncloth in each one on which was yellowness from menstrual blood, askingher about the prayer. She said to them, 'Do not be hasty until you seea white discharge." By that she meant purity from menses.
USC-MSA web (English) reference: Book 2, Hadith 99
Arabic reference: Book 2, Hadith 129