He who observes fasting during the month of Ramadan faithfully and expecting its reward from Allah will have his past sins forgiven. [Al-Bukhari]

"عن أبي هريرة -رضي الله عنه- مرفوعاً: «من صَام رمضان إيِمَانًا واحْتِسَابًا، غُفِر له ما تَقدَّم من ذَنْبِه». [صحيح.] - [متفق عليه.]"