Al-Kāfi

Hadith 258
fasting
6ـ عَلِيُّ بْنُ إِبْرَاهِيمَ عَنْ أَبِيهِ عَنِ ابْنِ أَبِي عُمَيْرٍ عَنْ سَلَمَةَ صَاحِبِ السَّابِرِيِّ عَنْ أَبِي الصَّبَّاحِ عَنْ أَبِي عَبْدِ الله (عَلَيْهِ السَّلاَم) قَالَ إِنَّ الله تَبَارَكَ وَتَعَالَى يَقُولُ الصَّوْمُ لِي وَأَنَا أَجْزِي عَلَيْهِ.
6. Ali ibn Ibrahim has narrated from his father from Ibn Abi ‘Umayr from Salamah, Sahib al-Sabiriy from abu al-Sabbah who has said the following: “Abu ‘Abd Allah (a.s.), has said that Allah, the most Blessed, the most High, has said, ‘Fasting is for Me, and I reward for it.’”
Source: Shaykh Muḥammad b. Yaʿqūb al-Kulaynī