Salim (may Allah have mercy on him) says that Ibn Umar (may Allah be pleased with them both) was asked about tamattu' Hajj, and he ordered that tamattu' be performed. It was said to him: You are contradicting your father. He said: My honoured father did not say what you say. He only said to separate Umrah from Hajj, meaning that Umrah in the months of Hajj is not complete without a sacrificial animal, and he wanted people to come to visit the House of Allah outside the months of Hajj as well. But you have declared it forbidden and begun punishing people for it, even though Allah has made it lawful and the Messenger of Allah (peace and blessings be upon him) acted upon it. When people raised many objections, he would say: Is the Book of Allah more deserving to be followed, or Umar (may Allah be pleased with him)?
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 8875