Musnad Ahmad 1034:88

Abu al-Mughirah narrated to us, Safwan narrated to us, al-Safar ibn Nusayr al-Azdi and others narrated to us, from Hudhayfah ibn al-Yaman that he said: O Messenger of Allah, we were in evil, and Allah took away that evil and brought good at your hands. So is there any evil after this good? He said: "Yes." He said: What is it? He said: "Tribulations like pieces of the dark night, following one another. They will come to you ambiguous, like the faces of cows; you will not know which is which."