Hadith: No fasting counts for the one who fasts continuously. Fasting three days is like fasting the entire year

عَنْ عَبْدَ اللَّهِ بْنِ عَمْرِو بْنِ العَاصِ رَضِيَ اللَّهُ عَنْهُمَا، قَالَ: قَالَ النَّبِيُّ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ:
«إِنَّكَ لَتَصُومُ الدَّهْرَ، وَتَقُومُ اللَّيْلَ؟»، فَقُلْتُ: نَعَمْ، قَالَ: «إِنَّكَ إِذَا فَعَلْتَ ذَلِكَ هَجَمَتْ لَهُ العَيْنُ، 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».
[صحيح] - [متفق عليه] - [صحيح البخاري: 1979]

‘Abdullah ibn ‘Amr ibn al-‘Ās (may Allah be pleased with him and his father) reported: The Prophet (may Allah's peace and blessings be upon him) said:
"You fast continuously and stand in prayer at night?" I said: Yes. He said: "If you do that, your eyes will become weak, and your soul will become weary. No fasting counts for the one who fasts continuously. Fasting three days is like fasting the entire year". I said: Indeed, I am capable of more than that. He said: "Then fast the fast of Dāwūd (David) (peace be upon him); he used to fast one day and break his fast the next day, and he would not flee when facing the enemy."
[Authentic hadith] - [Narrated by Bukhari & Muslim] - [Sahih Bukhari - 1979]

Explanation

The Prophet (may Allah's peace and blessings be upon him) was informed that ‘Abdullah ibn ‘Amr (may Allah be pleased with him and his father) was fasting continuously without breaking his fast throughout the year and was praying the entire night without sleeping. So, he forbade him from doing that and said to him: Fast and break your fast, and stand in prayer and sleep. He forbade him from fasting continuously and standing in prayer all night, and said to him: If you do that, your eyes will become weak and sunken, and your soul will become exhausted and weary. The one who fasts the entire year has not truly fasted, as he does not obtain the reward of fasting due to violating the prohibition, and he does not experience the benefit of breaking the fast, as he is constantly abstaining. Then he guided him to fasting three days of each month, as it is equivalent to fasting the entire year, because each day is counted as ten days, which is the minimum multiplication of a good deed. ‘Abdullah said: Indeed, I am capable of more than that. He (may Allah's peace and blessings be upon him) said: Then fast the fast of David (peace be upon him), which is the best type of fast, as he used to fast every other day and he would not flee when facing the enemy because the way he adopted in fasting did not weaken his body.