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

4ـ مُحَمَّدُ بْنُ يَحْيَى عَنْ أَحْمَدَ بْنِ مُحَمَّدٍ عَنْ عَلِيِّ بْنِ الْحَكَمِ عَنْ مُوسَى بْنِ بَكْرٍ قَالَ لِكُلِّ شَيْ‏ءٍ زَكَاةٌ وَزَكَاةُ الأَجْسَادِ الصَّوْمُ.

4. Muhammad ibn Yahya has narrated from Ahmad ibn Muhammad from Ali ibn al-Hakam from Musa ibn Bakr who has said the following: “He (the Imam), ‘Alayhi al-Salam, said, ‘Everything has a cleanser; the cleanser for bodies is fasting.”