Abu Qatadah (May Allah be pleased with him) reported:
The Messenger of Allah (ﷺ) was asked about the observance of Saum (fasting) on the day of 'Arafah. He said, "It is an expiation for the sins of the preceding year and the current year."[Muslim]. (DE)
App reference (DE): Book 9, Hadith 260 (DE)
Arabic/English book reference (DE): Book 9, Hadith 1250 (DE)