وَ قَالَ عَلِيٌّ ع قَالَ رَسُولُ اللَّهِ ص مَنْ صَامَ يَوْماً تَطَوُّعاً أَدْخَلَهُ اللَّهُ عَزَّ وَ جَلَّ الْجَنَّةَ.
English translationBab Ul Qaim Publications
Hadith.1801 - Commander of the Faithful Imam Ali ibn Abi Talib (as) said that the Messenger of Allah (swt) (peace and blessings be upon him and his family) said: "Whoever fasts a day voluntarily, Allah (swt), the Almighty, will admit him to Paradise."
Translation: Bab Ul Qaim Publications·Thaqalayn link·The Arabic above is the original.