Musnad Ahmad 27:1506

Al-Fadl ibn Dukayn narrated to us, Ibn Abi Rawwad narrated to us, from Nafi’, from Ibn Umar, that the Messenger of Allah (peace and blessings of Allah be upon him) used to touch the Yemeni Corner and the Black [Stone] in every circumambulation, and he would not touch the other two corners which are adjacent to the Hijr.