Musnad Ahmad 920
It was narrated that Umm Moosa said:
I heard `Ali (رضي الله عنه) say: The Prophet (ﷺ) instructed lbn Mas’ood to climb up a tree and he told him to bring him something from it, and his Companions looked at the shins of ‘Abdullah bin Masood when he climbed the tree and laughed at how thin his shins were, The Messenger of Allah (ﷺ) said: “Why are you laughing? The leg of ‘Abdullah will be heavier in the Balance on the Day of Resurrection than (Mount) Uhud.”
حَدَّثَنَا مُحَمَّدُ بْنُ فُضَيْلٍ، حَدَّثَنَا مُغِيرَةُ، عَنْ أُمِّ مُوسَى، قَالَتْ سَمِعْتُ عَلِيًّا، رَضِيَ اللَّهُ عَنْهُ يَقُولُ أَمَرَ النَّبِيُّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ ابْنَ مَسْعُودٍ فَصَعِدَ عَلَى شَجَرَةٍ أَمَرَهُ أَنْ يَأْتِيَهُ مِنْهَا بِشَيْءٍ فَنَظَرَ أَصْحَابُهُ إِلَى سَاقِ عَبْدِ اللَّهِ بْنِ مَسْعُودٍ حِينَ صَعِدَ الشَّجَرَةَ فَضَحِكُوا مِنْ حُمُوشَةِ سَاقَيْهِ فَقَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ مَا تَضْحَكُونَ لَرِجْلُ عَبْدِ اللَّهِ أَثْقَلُ فِي الْمِيزَانِ يَوْمَ الْقِيَامَةِ مِنْ أُحُدٍ‏.‏