38 - It was narrated that Abu Hurayrah said: The Messenger of Allah (blessings and peace of Allah be upon him) said: “Whoever fasts Ramadan, out of faith and in the hope of reward, his previous sins will be forgiven.”.

Commentary : This hadith brings great glad tidings from the Prophet (blessings and peace of Allah be upon him) to the one who is helped to fast the entire month of Ramadan when he is able to do that. Fasting means refraining, with the intention of worship, from eating, drinking, sexual activity and all other things that break the fast, from the break of dawn until the sun sets. Whoever fasts this month “out of faith and in the hope of reward”, believing in the divine command to do it, knowing that it is obligatory, out of fear of punishment for failing to do it, and seeking great reward for his fast – and this is a description of the believer – it is hoped that Allah will forgive him his previous sins, except those that involved transgressions against other people affecting their wealth, honour or physical well-being. Such sins cannot be waived except with their consent, so the person who transgressed against them must seek pardon from those to whom he owes something, or restore their dues.
The reward (forgiveness) is mentioned in the past tense [in the original Arabic], even though the forgiveness will come in the future, in order to give the sense that it will certainly happen and will definitely take place, by the grace of Allah (may He be exalted) to His slaves.
This hadith urges us to fast the month of Ramadan, and highlights the great reward for doing that..