(a) Sayyiduna Abdullah bin Buraydah narrates from his father (may Allah be pleased with him) that al-Najashi (the Negus) gave the Prophet (peace and blessings be upon him) a pair of plain black khuffs (leather socks) as a gift; he (peace and blessings be upon him) wore them and wiped over them.
(b) In the hadith of Ubaydullah it is stated that al-Najashi gave the Prophet (peace and blessings be upon him) a pair of black khuffs as a gift; he (peace and blessings be upon him) performed ablution and wiped over them.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 1345