Voluntary Fasting - Section 2 Mishkat al-Masabih 2061
Translation
Muslim al-Qurashi said that he or someone else asked God's messenger about perpetual fasting, and he replied, “You have a duty to your family. Fast during Ramadan and the following month, and every Wednesday and Thursday. You will then have observed a perpetual fast."Abu Dawud and Tirmidhi transmitted it.