Musnad Abu Ya'la al-Mawsili 2:49

Ubayd Allah ibn Umar narrated to us, Abd al-Rahman ibn Mahdi narrated to us, from Hisham ibn Sa’d, from Zayd ibn Aslam, from his father, who said: I heard Umar ibn al-Khattab saying: "Why the *ramal* (brisk walking) and the uncovering of the shoulders, when Allah has established Islam and banished polytheism?" He then said: "Nevertheless, we will not abandon something we used to do during the time of the Messenger of Allah, may Allah bless him and grant him peace."