Alqamah says: I said to Aishah (may Allah be pleased with her): "Did the Messenger of Allah (peace and blessings be upon him) use to single out any days (for fasting)?" She said: "No. His practice was constant, and which of you is able to do what he was able to do?"
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8472