All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
The narration to which the questioner pointed to is reported by Al-Bukhari and narrated by 'Amr Ibn Maymoon about the story of the death of 'Umar . The narration reads: "…A young man came to 'Umar after he was stabbed and told him: "O leader of the believers, you have glad tidings from Allaah, as you accompanied the Prophet and you are a senior in Islam, and then you were appointed as a caliph and you ruled with justice, and finally, you are blessed with martyrdom." ... Then 'Umar said: "I wish I would meet Allaah breaking even." And when the young man departed away 'Umar noticed that his lower garment was touching the ground. So, he asked people to call him and said to him: "O son! Raise your garment as, it is cleaner to your garment and reflects consciousness of Allaah." [Al-Bukhari] The narration does not suggest that 'Umar got very angry. The narration is evidence that he was keen on adhering to the Sunnah and warning people against neglecting it, and he advised the young man even in this very last moments of his life.
For more details, please refer to Fataawa 81448, 86519 and 84967 on the ruling of wearing garments below the ankle.
Allaah Knows best.