Mustadrak al-Hakim — Hadith 9018

It is narrated from Abdullah bin Abi Mulaykah that a man asked Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) about the saying of Allah, the Mighty and Majestic: "And indeed a day with your Lord is like a thousand years of what you count." [Surah al-Hajj: 47] He said: "Who are you?" The man told him that he was a man from such-and-such a place. Then Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: "Then what of the day whose measure is fifty thousand years?" The man said: May Allah have mercy on you! I only asked you so that you would tell us. Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: "These are two days that Allah, the Mighty and Majestic, has mentioned in His Book; Allah knows best about them. We dislike saying anything about the Book of Allah without knowledge."
This hadith is sahih (authentic) on the condition of Imam Bukhari, but the two Shaykhs (Bukhari and Muslim) did not record it. This is the end of Kitab al-Ahwal (the Book of Terrors), and it is the last book of "al-Jami' al-Sahih al-Mustadrak", compiled by al-Hakim, Imam Abu Abdullah Muhammad bin Abdullah bin Muhammad bin Hamdawayh al-Hafiz (may Allah have mercy on him).