Sunan an-Nasa'i, Hadith 2402 ⓘ
It was narrated that Ibn Al-Malih said: "I entered with Zaid upon 'Abdullah bin Amr and he narrated: 'The Messenger of Allah was told about my fasting, so the entered upon me and I gave him an average-sized leather pillow that was stuffed with palm fibvers. He sat in the ground with the pillow between myself and him, and said: "Whill it not be sufficient for you to fast three days each months?" I said: O Messenger of Allah! He said: "O Messenger of Allah!" He said: "Eleven." I said: "O Messenger of Allah!" Then the Prophet said: "There is not fast better than the fast of Dawud, half of a lifetime, fasting one day and not next."
أَخْبَرَنَا زَكَرِيَّاءُ بْنُ يَحْيَى، قَالَ حَدَّثَنَا وَهْبُ بْنُ بَقِيَّةَ، قَالَ أَنْبَأَنَا خَالِدٌ، عَنْ خَالِدٍ، - وَهُوَ الْحَذَّاءُ - عَنْ أَبِي قِلاَبَةَ، عَنْ أَبِي الْمَلِيحِ، قَالَ دَخَلْتُ مَعَ أَبِيكَ زَيْدٍ عَلَى عَبْدِ اللَّهِ بْنِ عَمْرٍو فَحَدَّثَ أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم ذُكِرَ لَهُ صَوْمِي فَدَخَلَ عَلَىَّ فَأَلْقَيْتُ لَهُ وِسَادَةَ أَدَمٍ رَبْعَةً حَشْوُهَا لِيفٌ فَجَلَسَ عَلَى الأَرْضِ وَصَارَتِ الْوِسَادَةُ فِيمَا بَيْنِي وَبَيْنَهُ قَالَ " أَمَا يَكْفِيكَ مِنْ كُلِّ شَهْرٍ ثَلاَثَةُ أَيَّامٍ " . قُلْتُ يَا رَسُولَ اللَّهِ . قَالَ " خَمْسًا " . قُلْتُ يَا رَسُولَ اللَّهِ . قَالَ " سَبْعًا " . قُلْتُ يَا رَسُولَ اللَّهِ . قَالَ " تِسْعًا " . قُلْتُ يَا رَسُولَ اللَّهِ . قَالَ " إِحْدَى عَشْرَةَ " . قُلْتُ يَا رَسُولَ اللَّهِ . فَقَالَ النَّبِيُّ صلى الله عليه وسلم " لاَ صَوْمَ فَوْقَ صَوْمِ دَاوُدَ شَطْرَ الدَّهْرِ صِيَامُ يَوْمٍ وَفِطْرُ يَوْمٍ " .
also referenced under: Book 22, Hadith 313 (In-Book) & Volume 3, Book 22, Hadith 2404 (DarusSalam)