Musnad Ahmad 31:281

Waki' narrated to us, Al-A'mash narrated to us, from Abu Salih, from Abu Sa'id al-Khudri, from the Prophet, peace and blessings of Allah be upon him: "The 'Wasat' is the just (al-'adl)." [Regarding the verse]: "Thus We have made you a middle (wasat) nation" [Al-Baqarah: 143].