Salim says that Abdullah bin Umar (may Allah be pleased with them both) used to give people the fatwa of the concession of tamattu' that Allah Almighty has granted, and which is also the Sunnah of the Messenger of Allah (peace and blessings be upon him). People said to him: How can you contradict your father, when he forbids it? Abdullah bin Umar (may Allah be pleased with them both) said to them: Woe to you! Do you not fear Allah Almighty? Do you not understand that Umar (may Allah be pleased with him) forbade it seeking good and the completion of Umrah? So why do you declare it forbidden when Allah Almighty has declared it lawful and the Messenger of Allah (peace and blessings be upon him) acted upon it? Is following the Sunnah of the Messenger of Allah (peace and blessings be upon him) more necessary, or that of Umar (may Allah be pleased with him)? Umar (may Allah be pleased with him) did not say that Umrah in the months of Hajj is forbidden; rather he said that Umrah is completed in a better way when you perform it outside the months of Hajj.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 8876