Maliks Muwatta Book 2, Hadith Number 99.

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.