Sunan an-Nasa'i, Volume 3, Book 22, Hadith 2390 (DarusSalam) ⓘ

It was narrated that 'Abdullah bin 'Amr said: "The Messenger of Allah said: "The best of fasting is the fast of Dawud, peace be upon him. He used to fast for 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."

قَالَ وَفِيمَا قَرَأَ عَلَيْنَا أَحْمَدُ بْنُ مَنِيعٍ قَالَ حَدَّثَنَا هُشَيْمٌ، قَالَ أَنْبَأَنَا حُصَيْنٌ، وَمُغِيرَةُ، عَنْ مُجَاهِدٍ، عَنْ عَبْدِ اللَّهِ بْنِ عَمْرٍو، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏ "‏ أَفْضَلُ الصِّيَامِ صِيَامُ دَاوُدَ عَلَيْهِ السَّلاَمُ كَانَ يَصُومُ يَوْمًا وَيُفْطِرُ يَوْمًا ‏" ‏ ‏.‏