Ibn Umar reported: The Messenger of Allah, peace and blessings be upon him, said:

مَا مِنْ أَيَّامٍ أَعْظَمُ عِنْدَ اللَّهِ وَلَا أَحَبُّ إِلَيْهِ مِنْ الْعَمَلِ فِيهِنَّ مِنْ هَذِهِ الْأَيَّامِ الْعَشْرِ فَأَكْثِرُوا فِيهِنَّ مِنْ التَّهْلِيلِ وَالتَّكْبِيرِ وَالتَّحْمِيدِ

There are no days greater and more beloved to Allah than these ten days of Dhul Hijjah, so increase in them your declaration of the oneness of Allah (Laa ilaha ilal’Allah), your exaltation of Him (Allahu Akbar), and your praise of Him (Alhamdulillah).

Source: Musnad Aḥmad 5423, Grade: Sahih

Fasting on the day of Arafah will expiate the sins of the previous and upcoming years - Sahih Muslim 1162