Musannaf Ibn Abi Shayba 13:2912

Abu Bakr narrated to us, he said: Waki’ narrated to us, he said: Khalid bin Abd al-Rahman bin Bukayr al-Sulami narrated to us, from Ibn Sirin, who said: I witnessed Shurayh permit the testimony of a group of the people of shirk (polytheists) for one another, regarding their khuff (leather socks) that were stained.