The fasting of three days (a month) will be equal to..

Narrated `Abdullah bin `Amr bin Al-`As:

حَدَّثَنَا آدَمُ، حَدَّثَنَا شُعْبَةُ، حَدَّثَنَا حَبِيبُ بْنُ أَبِي ثَابِتٍ، قَالَ سَمِعْتُ أَبَا الْعَبَّاسِ الْمَكِّيَّ ـ وَكَانَ شَاعِرًا وَكَانَ لاَ يُتَّهَمُ فِي حَدِيثِهِ ـ قَالَ سَمِعْتُ عَبْدَ اللَّهِ بْنَ عَمْرِو بْنِ الْعَاصِ ـ رضى الله عنهما ـ قَالَ قَالَ لِي النَّبِيُّ صلى الله عليه وسلم ‏"‏ إِنَّكَ لَتَصُومُ الدَّهْرَ، وَتَقُومُ اللَّيْلَ ‏"‏‏.‏ فَقُلْتُ نَعَمْ‏.‏ قَالَ ‏"‏ إِنَّكَ إِذَا فَعَلْتَ ذَلِكَ هَجَمَتْ لَهُ الْعَيْنُ وَنَفِهَتْ لَهُ النَّفْسُ، لاَ صَامَ مَنْ صَامَ الدَّهْرَ، صَوْمُ ثَلاَثَةِ أَيَّامٍ صَوْمُ الدَّهْرِ كُلِّهِ ‏"‏‏.‏ قُلْتُ فَإِنِّي أُطِيقُ أَكْثَرَ مِنْ ذَلِكَ‏.‏ قَالَ ‏"‏ فَصُمْ صَوْمَ دَاوُدَ عَلَيْهِ السَّلاَمُ كَانَ يَصُومُ يَوْمًا وَيُفْطِرُ يَوْمًا، وَلاَ يَفِرُّ إِذَا لاَقَى ‏"‏‏.‏

The Prophet (ﷺ) said to me, "You fast daily all the year and pray every night all the night?" I replied in the affirmative. The Prophet (ﷺ) said, "If you keep on doing this, your eyes will become weak and your body will get tired. He who fasts all the year is as he who did not fast at all. The fasting of three days (a month) will be equal to the fasting of the whole year." I replied, "I have the power for more than this." The Prophet (ﷺ) said, "Then fast like the fasting of David who used to fast on alternate days and would never flee from the battle field, on meeting the enemy."

Reference : Sahih al-Bukhari 1979
In-book reference : Book 30, Hadith 86
USC-MSA web (English) reference : Vol. 3, Book 31, Hadith 200 (deprecated numbering scheme)