In the Name of Allāh, the Ever Merciful, the Bestower of Mercy
It has been authentically attributed to Prophet ﷺ, that he said, “Fasting on the day of ʿĀrafah expiates the sins of two years [i.e. previous year and upcoming year] and fasting on ʿĀshūrāh expiates the sins of one year”.
But his saying, “...expiates the sins of...”, does not necessitate that the major sins are also expiated without any repentance. The Prophet ﷺ said, “...Friday to Friday and Ramaḍān to Ramaḍān will be an expiation for the sins committed between them as long as the major sins are avoided”.
It is known that prayer is better than fasting and that fasting in Ramaḍān is better than fasting the day of ʿArafah. Likewise, (minor) sins will not be expiated except by avoiding the major sins as the Prophet ﷺ mentioned. Therefore, how can an individual think that by voluntarily fasting a day or two it will expiate adultery, stealing, drinking alcohol, gambling, (engaging in) magic, etc. This will not happen.
Source: [١/٢٥٤ مختصر الفتاوى المصرية]
Translated by: Munīb al-Sumālī