Sahih al-Bukhari 1976

أُخْبِرَ رَسُولُ اللَّهِ صلى الله عليه وسلم أَنِّي أَقُولُ وَاللَّهِ لأَصُومَنَّ النَّهَارَ، وَلأَقُومَنَّ اللَّيْلَ، مَا عِشْتُ‏.‏ فَقُلْتُ لَهُ قَدْ قُلْتُهُ بِأَبِي أَنْتَ وَأُمِّي‏.‏ قَالَ ‏"‏ فَإِنَّكَ لاَ تَسْتَطِيعُ ذَلِكَ، فَصُمْ وَأَفْطِرْ، وَقُمْ وَنَمْ، وَصُمْ مِنَ الشَّهْرِ ثَلاَثَةَ أَيَّامٍ، فَإِنَّ الْحَسَنَةَ بِعَشْرِ أَمْثَالِهَا، وَذَلِكَ مِثْلُ صِيَامِ الدَّهْرِ ‏"‏‏.‏ قُلْتُ إِنِّي أُطِيقُ أَفْضَلَ مِنْ ذَلِكَ‏.‏ قَالَ ‏"‏ فَصُمْ يَوْمًا وَأَفْطِرْ يَوْمَيْنِ ‏"‏‏.‏ قُلْتُ إِنِّي أُطِيقُ أَفْضَلَ مِنْ ذَلِكَ‏.‏ قَالَ ‏"‏ فَصُمْ يَوْمًا وَأَفْطِرْ يَوْمًا، فَذَلِكَ صِيَامُ دَاوُدَ ـ عَلَيْهِ السَّلاَمُ ـ وَهْوَ أَفْضَلُ الصِّيَامِ ‏"‏‏.‏ فَقُلْتُ إِنِّي أُطِيقُ أَفْضَلَ مِنْ ذَلِكَ‏.‏ فَقَالَ النَّبِيُّ صلى الله عليه وسلم ‏"‏ لاَ أَفْضَلَ مِنْ ذَلِكَ ‏"‏‏.

“Allah's Messenger ﷺ was informed that I had taken an oath to fast daily and to pray (every night) all the night throughout my life (so Allah's Messenger ﷺ came to me and asked whether it was correct): I replied, "Let my parents be sacrificed for you! I said so." The Prophet ﷺ said, "You can not do that. So, fast for few days and give it up for few days, offer Salat (prayer) and sleep. Fast three days a month as the reward of good deeds is multiplied ten times and that will be equal to one year of fasting." I replied, "I can do better than that." The Prophet ﷺ said to me, "Fast one day and give up fasting for two days." I replied, "I can do better than that." The Prophet ﷺ said to me, "Fast one day and give up fasting for a day and that is the fasting of Prophet David and that is the best fasting." I said, "I have the power to fast better (more) than that." The Prophet ﷺ said, "There is no better fasting than that”