Sayyiduna Ibn Umar (may Allah be pleased with them both) said that the Prophet (peace and blessings be upon him) said: "Whoever matted his hair should shave his head; shaving the head is obligatory upon him."
Asim bin Umar is da'if (weak), and this is not established as a saying of the Prophet (peace and blessings be upon him) (marfu').
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 9588