حَدَّثَنِي يَحْيَى، عَنْ مَالِكٍ، عَنْ عَلْقَمَةَ بْنِ أَبِي عَلْقَمَةَ، عَنْ أُمِّهِ، مَوْلاَةِ عَائِشَةَ أُمِّ الْمُؤْمِنِينَ أَنَّهَا قَالَتْ كَانَ النِّسَاءُ يَبْعَثْنَ إِلَى عَائِشَةَ أُمِّ الْمُؤْمِنِينَ بِالدِّرَجَةِ فِيهَا الْكُرْسُفُ فِيهِ الصُّفْرَةُ مِنْ دَمِ الْحَيْضَةِ يَسْأَلْنَهَا عَنِ الصَّلاَةِ فَتَقُولُ لَهُنَّ لاَ تَعْجَلْنَ حَتَّى تَرَيْنَ الْقَصَّةَ الْبَيْضَاءَ ‏.‏ تُرِيدُ بِذَلِكَ الطُّهْرَ مِنَ الْحَيْضَةِ ‏.‏

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. Translation Not Available

Book Ref: Muwatta Imam Malik Book 2 Hadith 129
Web Ref:  Muwatta Imam Malik Book 2 Hadith 99