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].

وعن أبي قتادة رضي الله عنه قال: سئل رسول الله ﷺ: عن صوم يوم عرفة؟ قال: "يكفر السنة الماضية والباقية" ((رواه مسلم)).