Abdullah bin Abi Musa asked Sayyidah Aishah (may Allah be pleased with her) about fasting on the day about which there is doubt. She said: Fasting a day of Sha'ban is dearer to me than leaving a fast of Ramadan.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 7971
Sayyidah Fatimah bint al-Mundhir (may Allah have mercy on her) narrated from Sayyidah Asma (may Allah be pleased with her) that she used to fast the day of doubt about Ramadan.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 7972