Musnad Abu Ya'la al-Mawsili 251:3

Mukhlad ibn Abi Zumayl narrated to us, Ubayd Allah ibn Amr narrated to us, from Zayd ibn Abi Anisah, from Abu Ishaq al-Subay’i, from Jarir ibn Abdullah al-Bajali, from the Prophet (may Allah bless him and grant him peace), who said: "Fasting three days of every month is fasting for a lifetime. [They are] the white days: the morning of the thirteenth, the fourteenth, and the fifteenth."