The noble Shaykh Muhammad Ibn Hizaam mentions in his amazing book Itihaaful Anaam bi Ahkaam Wa Masaaail As Siyaam pg 159 Maktabatul Falaah:

“It has come in Saheeh Muslim from the narration of Abu Qatadah radiallahu anhu that the prophet Sallallahu alaihi wassallam was asked about the fasting on the day of Arafat. So he said: ‘It expiates the sins of the previous year and that of the following year’

In this narration is clear proof upon the recommendation of fasting the day of Arafat and it is the 9th day of Thul Hijjah

Abdullah Ibn Bassam, may Allah grant him success said: The fasting of the day of Arafat is the most virtuous of the voluntary fasts by consensus of the scholars

[Tawdeehul Ahkaam 3/201]

As Sanaani said in As Subul: it has become problematic the expiation of that which has not occurred and it is the sins of the following year. It is replied with that the intent [of that is] that he would be given success in it [i.e. the year] via not committing the sin, and he named it an expiation due to that which passed. Or, [it means] if it were to occur in it a sin, then he would be given success to doing that which will expiate it.

[Subulus Salam 2/339 Dar kitabul Arab]”

Translated by

Majid Jawed Al-Afghanee
Abu Layl