Al-Kāfi - Volume 4 > The Excellence of Fast and Fasting
Hadith #3

3ـ مُحَمَّدُ بْنِ يَحْيَى عَنْ أَحْمَدَ بْنِ مُحَمَّدِ بْنِ عِيسَى عَنِ ابْنِ فَضَّالٍ عَنْ ثَعْلَبَةَ عَنْ عَلِيِّ بْنِ عَبْدِ الْعَزِيزِ قَالَ قَالَ لِي أَبُو عَبْدِ الله (عَلَيْهِ السَّلاَم) أَ لا أُخْبِرُكَ بِأَصْلِ الإِسْلامِ وَفَرْعِهِ وَذِرْوَتِهِ وَسَنَامِهِ قُلْتُ بَلَى قَالَ أَصْلُهُ الصَّلاةُ وَفَرْعُهُ الزَّكَاةُ وَذِرْوَتُهُ وَسَنَامُهُ الْجِهَادُ فِي سَبِيلِ الله أَ لا أُخْبِرُكَ بِأَبْوَابِ الْخَيْرِ إِنَّ الصَّوْمَ جُنَّةٌ.

3. Muhammad ibn Yahya has narrated from Ahmad ibn Muhammad ibn ‘Isa from ibn Faddal from Tha’labah from Ali ibn ‘Abd al-‘Aziz who has said the following: “Abu ‘Abd Allah, ‘Alayhi al-Salam, once asked me, ‘Should I tell you what the root, the branch, the peak and the highest point of Islam is?’ I replied, ‘Please, do so.’ He said, ‘The root of Islam is salat (prayer), its branch is zakat, and its peak and highest point is al-Jihad in the way of Allah. Should I tell you about the doors to goodness? Fasting certainly is a shield.’”