(77) باب ذِكْرِ الزِّيَادَةِ فِي الصِّيَامِ وَالنُّقْصَانِ وَذِكْرِ اخْتِلاَفِ النَّاقِلِينَ لِخَبَرِ عَبْدِ اللَّهِ بْنِ عَمْرٍو فِيهِ
It was narrated from 'Abdullah bin 'Amr that the Messenger of Allah (ﷺ) said to him:
"Fast one day, and you will have the reward of what is left." He said: "I am able to do more than that." He (ﷺ) said: "Fast two days, and you will have the reward of what is left." He said: "I am able to do more than that." He (ﷺ) said: "Fast three days and you will have the reward of what is left." He said: "I am able to do more than that." He (ﷺ) said: "Fast four days and you will have the reward of what is left." He said: "I am able to do more than that." He (ﷺ) said: "Observe the best kind of fasting before Allah, the fast of Dawud, peace be upon him; he used to fast one day and break his fast for one day." 'Ata said: "Someone who heard him told me that Ibn 'Umar (said) that the Prophet said: 'Whoever fasts every day of his life, then he has not fasted.'"
Reference: Sunan an-Nasa'i 2394
In-book reference: Book 22, Hadith 305
English translation: Vol. 3, Book 22, Hadith 2396