Musannaf Ibn Abi Shaybah 9982 — Fasting the day of Arafah when not

Humayd al-Tawil said: Once it was mentioned in the presence of al-Hasan that the reward of fasting on the Day of Arafah is equal to fasting for a year. On hearing this, al-Hasan said: "As far as I know, no day has any superiority over another day, and no night has any superiority over another, except Laylat al-Qadr. Laylat al-Qadr is better than a thousand months. I saw Sayyiduna Uthman bin Abi al-As (may Allah be pleased with him) fasting on the Day of Arafah, and water would be sprinkled on him to cool him because of the intense heat."