It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that when he wiped his head, he would also wipe his nape along with his head.
(b) This report is mawquf (stopping at the Companion), and the musnad (attributed to the Prophet) version has weakness in its chain. And Allah knows best.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 279