Musnad Ahmad 27:1432

Ibrahim narrated to us, Ibn al-Mubarak narrated to us, from Abu al-Sabah al-Ayli, who said: I heard Yazid ibn Abi Sumayya saying: I heard Ibn Umar saying: "Whatever the Messenger of Allah, may Allah bless him and grant him peace, said regarding the lower garment (izar), it applies to the shirt (qamis)."