I asked or someone asked the Prophet ﷺ about perpetual fasting. 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.  

سَأَلْتُ أَوْ سُئِلَ النَّبِيُّ ﷺ عَنْ صِيَامِ الدَّهْرِ فَقَالَ إِنَّ لأَهْلِكَ عَلَيْكَ حَقًّا صُمْ رَمَضَانَ وَالَّذِي يَلِيهِ وَكُلَّ أَرْبِعَاءَ وَخَمِيسٍ فَإِذَا أَنْتَ قَدْ صُمْتَ الدَّهْرَ