Musnad Ahmad 3:56

Abdullah narrated to us, Yahya ibn Ma'in narrated to me, Hisham ibn Yusuf narrated to us, Abdullah ibn Buhayr al-Qass narrated to me, from Hani, the freed slave of Uthman, who said: When Uthman would stand over a grave, he would weep until his beard became wet. It was said to him: "Paradise and Hell are mentioned and you do not weep, yet you weep because of this?" He said: "Indeed, the Messenger of Allah (peace and blessings of Allah be upon him) said: 'The grave is the first of the stages of the Hereafter. If one is saved from it, then what comes after it is easier than it; and if one is not saved from it, then what comes after it is more severe than it.'" He said: And the Messenger of Allah (peace and blessings of Allah be upon him) said: "I have never seen a sight at all, except that the grave is more horrific than it."