Muwatta Malik, Book 2, Hadith 102 (USC-MSA)

Yahya related to me from Malik that he had heard that A'isha, the wife of the Prophet, may Allah bless him and grant him peace, said that a pregnant woman who noticed bleeding left off from prayer.

حَدَّثَنِي يَحْيَى، عَنْ مَالِكٍ، أَنَّهُ بَلَغَهُ أَنَّ عَائِشَةَ، زَوْجَ النَّبِيِّ صلى الله عليه وسلم قَالَتْ فِي الْمَرْأَةِ الْحَامِلِ تَرَى الدَّمَ أَنَّهَا تَدَعُ الصَّلاَةَ ‏.‏

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also referenced under: Book 2, Hadith 131 (In-Book)