Muwatta Malik 128
Purity
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 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
Grading: Salim al-Hilali: Mauquf Sahih Lighairihi
Text from the public-domain hadith-api dataset. Numbering follows that dataset and can differ by a few from some printed editions — check the wording, not only the number.