Hadith 4:856
Yazid ibn Harun narrated to us, he said: Humayd al-Tawil narrated to us, he said: It was mentioned in the presence of al-Hasan that the fasting of Arafah is equivalent to the fasting of a year. Al-Hasan said: I do not know of any day having superiority over another day, nor any night over another night, except for the Night of Decree (Laylat al-Qadr), for it is better than a thousand months. And I have seen Uthman ibn Abi al-As fasting on the day of Arafah, while water was being sprinkled upon him from a water skin he had with him, to cool himself with it.
حدثنا يزيد بن هارون قال حدثنا حميد الطويل قال ذكر عند الحسن أن صيام عرفة يعدل صيام سنة فقال الحسن ما أعلم ليوم فضلا على يوم ولا لليلة على ليلة إلا ليلة القدر فإنها خير من ألف شهر ولقد رأيت عثمان بن أبي العاص صام يوم عرفة يرش عليه الماء من إداوة معه يتبرد به