Sahih al-Bukhari 38
Sahih al-Bukhari · Book 2: Belief · To observe fasting (according to Islamic teachings)] during the month of Ramadan (sincerely and faithfully) hoping for Allah's Reward only, is a part of faith
Narrated Abu Huraira:
حَدَّثَنَا ابْنُ سَلاَمٍ ، قَالَ أَخْبَرَنَا مُحَمَّدُ بْنُ فُضَيْلٍ ، قَالَ حَدَّثَنَا يَحْيَى بْنُ سَعِيدٍ ، عَنْ أَبِي سَلَمَةَ ، عَنْ أَبِي هُرَيْرَةَ ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " مَنْ صَامَ رَمَضَانَ إِيمَانًا وَاحْتِسَابًا غُفِرَ لَهُ مَا تَقَدَّمَ مِنْ ذَنْبِهِ " .
Allah's Messenger (ﷺ) said, "Whoever observes fasts during the month of Ramadan out of sincere faith, and hoping to attain Allah's rewards, then all his past sins will be forgiven."