Musnad Ahmad 454
It was narrated that Hani`, the freed slave of `Uthman, (رضي الله عنه) said:
`Uthman (رضي الله عنه) used to stand by a grave and weep until his beard became wet. It was said to him: You remember Paradise and Hell and you do not weep, but you weep for this? He said: The Messenger of Allah (ﷺ) said: `The grave is the first stage of the hereafter. If one is saved from it (i.e., its torments), then what comes after it is easier than it. But if one is not saved from it {i.e. its torments), then what comes after it is worse.` And the Messenger of Allah (ﷺ)  said: By Allah, I have never seen any frightening scene but the grave is more frightening than it.`
حَدَّثَنَا عَبْد اللَّهِ، حَدَّثَنَا يَحْيَى بْنُ مَعِينٍ، حَدَّثَنَا هِشَامُ بْنُ يُوسُفَ، حَدَّثَنِي عَبْدُ اللَّهِ بْنُ بَحِيرٍ الْقَاصُّ، عَنْ هَانِئٍ، مَوْلَى عُثْمَانَ رَضِيَ اللَّهُ عَنْهُ قَالَ كَانَ عُثْمَانُ رَضِيَ اللَّهُ عَنْهُ إِذَا وَقَفَ عَلَى قَبْرٍ بَكَى حَتَّى يَبُلَّ لِحْيَتَهُ فَقِيلَ لَهُ تَذْكُرُ الْجَنَّةَ وَالنَّارَ فَلَا تَبْكِي وَتَبْكِي مِنْ هَذَا فَقَالَ إِنَّ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قَالَ الْقَبْرُ أَوَّلُ مَنَازِلِ الْآخِرَةِ فَإِنْ يَنْجُ مِنْهُ فَمَا بَعْدَهُ أَيْسَرُ مِنْهُ وَإِنْ لَمْ يَنْجُ مِنْهُ فَمَا بَعْدَهُ أَشَدُّ مِنْهُ قَالَ وَقَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ وَاللَّهِ مَا رَأَيْتُ مَنْظَرًا قَطُّ إِلَّا وَالْقَبْرُ أَفْظَعُ مِنْهُ‏.‏