Hani reported,
Whenever Uthman 
stood at a grave, he would weep until his beard was soaked. Someone said to him, "You remember Paradise and the Fire and do not weep, yet you weep at this?"He replied, "The Messenger of Allah ﷺ said, 'The grave is the first of the stages of the Hereafter. Whoever is delivered from it, what comes after is easier, and whoever is not delivered from it, what comes after is harsher.'"He continued, "And the Messenger of Allah ﷺ said, 'By Allah, I have never seen any sight more dreadful than the grave.'"
Ahmad 454
Sahihحدثنا عبد الله حدثنا يحيى بن معين حدثنا هشام بن يوسف حدثني عبد الله بن بحير القاص
عن هانئ مولى عثمان رضي الله عنه قال
كان عثمان رضي الله عنه إذا وقف على قبر بكى حتى يبل لحيته فقيل له تذكر الجنة والنار فلا تبكي وتبكي من هذا فقال إن رسول الله ﷺ قال القبر أول منازل الآخرة فإن ينج منه فما بعده أيسر منه وإن لم ينج منه فما بعده أشد منه قال وقال رسول الله ﷺ والله ما رأيت منظرا قط إلا والقبر أفظع منه
Its isnad is sahih. Hisham bin Yusuf is the Sanani, the Abnawi, judge of Sanaa, and he is trustworthy and precise. In H it appears as Hisham bin Yunus, which is an error, and we have corrected it from K and H.Abdullah bin Buhayr, with fatha on the ba and kasra on the ha, ibn Raysan, with fatha on the ra and sukoon on the ya and with a plain sin, the Muradi, the Qasi, the Sanani, was deemed trustworthy by Ibn Main and others.Hani the Berber, freedman of Uthman, is trustworthy.The hadith was narrated by Tirmidhi 3/258, who said, hasan gharib, we do not know it except from the hadith of Hisham bin Yusuf. It was also narrated by Ibn Majah 2/294 and by Hakim in Mustadrak 1/371.This hadith is among the additions of Abdullah bin Ahmad.