Musnad Ahmad 2:156

'Abd al-Razzaq narrated to us, 'Ubayd Allah ibn 'Umar informed us, from Nafi', who said: Ibn 'Umar saw Sa'd ibn Malik wiping over his leather socks (khuffayn), so Ibn 'Umar said: "And do you really do this?" Sa'd said: "Yes." They met with 'Umar, and Sa'd said: "O Commander of the Faithful, give a legal opinion to my nephew regarding wiping over the leather socks." 'Umar said: "We used to wipe over our leather socks while we were with our Prophet, may Allah bless him and grant him peace." Ibn 'Umar said: "Even if he comes from defecation and urination?" 'Umar said: "Yes, even if he comes from defecation and urination." Nafi' said: After that, Ibn 'Umar used to wipe over them as long as he did not take them off, and he did not set a specific time limit for that. I narrated it to Ma'mar, and he said: Ayyub narrated it to me, from Nafi', the like of it.