Musnad Ahmed: Hadith 1945

The last hardship the believer faces is death, And he said concerning the verse, "The Day that the sky will be like al-muhl." [al-Ma"arij 70:8]: [This means] the fifth of oil. And he said concerning the verse, “during the hours of the night" [Al-‘Imran 3:113]. This means] in the depths of the night. And he said: Do you know what the disappearance of knowledge is? He said: It is the disappearance of the scholars from the earth.