My friend (i.e. the Prophet) instructed me to observe three practices that I do not leave while traveling nor while resident, to pray two rakʿahs in the forenoon, to fast three days every month and not to sleep but after observing the witr. (Using translation from Abū Dāʾūd 1432)
أَوْصَانِي خَلِيلِي أَبُو الْقَاسِمِ ﷺ بِثَلَاثٍ لَا أَدَعُهُنَّ لِشَيْءٍ أَوْصَانِي بِصِيَامِ ثَلَاثَةِ أَيَّامٍ مِنْ كُلِّ شَهْرٍ وَأَنْ لَا أَنَامَ إِلَّا عَلَى وِتْرٍ وَسُبْحَةِ الضُّحَى فِي الْحَضَرِ وَالسَّفَرِ