Musnad Ahmad 30:1585

Yahya ibn Abi Bukayr narrated to us, Zuhayr ibn Muhammad narrated to us, from Salih, the freed slave of al-Taw'amah, from Abu Hurayrah: "That the Prophet, peace and blessings of Allah be upon him, used to have the muscle of his calf visible from beneath his lower garment when he wore a lower garment."