When Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) finished the fasts of Ramadan while intending Hajj, he would not cut the hair of his head or his beard until he had performed Hajj.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 8947
Sayyiduna Jabir (may Allah be pleased with him) said: "We used to be commanded to let our hair grow in Hajj and Umrah." Al-Muharibi said: That is, on the Day of Sacrifice, at the time of shaving.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 8948