“من صام رمضان إيمانا ً واحتساباً غفر له ما تقدم من ذنبه”

“Whoever fasts the month of Ramadhan sincerely to Allah while seeking the reward of it will have his previous sins forgiven” [collected by Al-Bukhari and Muslim]

Shaykh Uthaymeen رحمه الله mentions;

“Meaning: [the fasting person] must be pleased with the obligation of fasting during Ramadhan while seeking the reward of his fast with his Lord, without having any dislike in his heart with its obligation or any doubt in its reward. If this happened then Allah will forgive his previous sins”. [¹]

[¹] “Sittings of Ramadhan” by Shaykh Uthaymeen مجالس شهر رمضان، المجلس الثاني