Yahya related to me from Malik from AIqama ibn Abi AIqama that
his mother, the mawla of A'isha, umm al-muminin, said, "Women used to
send little boxes to A'isha, umm al-muminin, with a piece of cotton
cloth in each one on which was yellowness from menstrual blood, asking
her about the prayer. She said to them, 'Do not be hasty until you see
a white discharge." By that she meant purity from menses.
Muwatta Malik Book 2, Hadith 98