Musnad Ahmad 2:220

Yazid narrated to us, Asim informed us, from Abu Uthman al-Nahdi, from Umar ibn al-Khattab, that he said: Wear the izar (waist-wrapper) and the rida (upper garment), wear sandals, and cast away the khuffayn (leather socks) and the sarawilat (trousers). Expose the knees and jump (in practice/exercise). Adhere to the Ma'addiyyah (the way of the tribe of Ma'add, implying a rugged, desert lifestyle), shoot at targets, and abandon luxury and the attire of the non-Arabs. Beware of silk, for the Messenger of Allah (peace and blessings of Allah be upon him) forbade it and said: "Do not wear of silk except what is like this," and the Messenger of Allah (peace and blessings of Allah be upon him) gestured with his two fingers.