Musnad Abu Ya'la al-Mawsili 154:11

Abu al-Walid al-Qurashi narrated to us, al-Walid ibn Muslim narrated to us, Zuhayr ibn Muhammad informed me, from Zayd ibn Aslam, that he informed him: That he used to see Ibn Umar with the button of his shirt undone. He was asked about that, and he said: "I saw the Prophet, may Allah bless him and grant him peace, doing it."