Musnad Ahmad 27:802

Waki’ narrated to us, al-‘Umari narrated to us, from Sa’id al-Maqburi and Nafi’, that Ibn ‘Umar used to wear al-sabtiyyah (sandals made of tanned leather) and perform ablution in them. And he mentioned that the Prophet, peace and blessings of Allah be upon him, used to do that.