Musnad Ahmad 1034:41

Yazid ibn Harun narrated to us, Abu Malik narrated to us, from Rib'i ibn Hirash, from Hudhayfah, that he arrived from Umar’s presence. He said: When we sat with him yesterday, he asked the companions of Muhammad, peace and blessings of Allah be upon him: "Which of you heard the statement of the Messenger of Allah, peace and blessings of Allah be upon him, regarding the tribulations (fitan)?" They said: "We heard it." He said: "Perhaps you mean the tribulation of a man regarding his family and his wealth?" They said: "Yes." He said: "I am not asking about that; that is expiated by prayer, fasting, and charity. But which of you heard the statement of the Messenger of Allah, peace and blessings of Allah be upon him, regarding the tribulations that surge like the surging of the sea?" He said: The people remained silent, and I thought he meant me. I said: "I did." He said to me: "May your father be for Allah (an expression of praise/encouragement), you!" I said: "Tribulations are presented to the hearts, mat by mat (layer by layer). Any heart that rejects them, a white dot is marked in it; and any heart that absorbs them, a black dot is marked in it, until the heart becomes one of two types: white like a smooth stone (safa), which no tribulation will harm as long as the heavens and the earth remain; and the other, black and dusty, like an overturned cup—and he tilted his palm—it does not recognize any good, nor does it reject any evil, except for what it has absorbed of its own desires."