`Ali narrates - Allah be well-pleased with him - that Allah's Messenger - Allah bless and greet him - summoned Ibn Mas`ud one day. He [came and] climbed a tree to get something for him. His companions looked at `Abd Allah's leg and laughed at the thinness (humusha) of his legs. Whereupon Allah's Messenger said -- Allah bless and greet him: "Why are you laughing? I swear that `Abd Allah's foot shall weigh more heavily than [Mount] Uhud in the Balance on the Day of Judgment."
Narrated by al-Hakim in al-Mustadrak, Book of the Knowledge of the Companions (#5385). He declared it of sound authenticity (sahih) and al-Dhahabi concurred with him. It is also narrated from Qarra ibn Iyas al-Muzani in the following wording:
Ibn Mas`ud climbed a tree and they started laughing at the thinness of his legs, whereupon the Prophet said -- Allah bless and greet him: "I swear that they shall be heavier in the Balance than Mount Uhud."
Narrated by Ahmad in his Musnad (#920), Abu Ya`la in his (#539), al-Tabarani in al-Kabir (38516), Ibn Sa`d in his Tabaqat (3:109), Abu Nu`aym in Hilya al-Awliya (1:127), al-Hakim, and al-Haythami indicated that its chain was sound in Majma` al-Zawa'id (#15561).
Allah's blessings and peace on the Prophet, his Family, and all his Companions.