Musnad Ahmad 25:1089

From ‘Abd al-Samad, from his father, from al-Hasan—meaning Ibn Dhakwan—from Habib, from Sa‘id ibn Jubayr, from Ibn ‘Abbas, that the Prophet (peace and blessings of Allah be upon him) forbade walking in one khuff (leather sock) or one sandal. And in the hadith there is much speech other than this, so he did not narrate it to us; he struck through it in his book. I thought that he abandoned his hadith because he narrated from ‘Amr ibn Khalid, who narrates from Zayd ibn ‘Ali, and ‘Amr ibn Khalid is worth nothing.