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 for ihram, shaving the head has become obligatory upon him."
This Abdullah bin Nafi' is not strong; what is correct is that it is the saying of Umar and Ibn Umar (may Allah be pleased with them both), and likewise Salim narrated it from his father, from Umar.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 9583