Salim (may Allah have mercy on him) says that a person asked Sayyiduna Ibn Umar (may Allah be pleased with them both) about Hajj Tamattu', so he permitted it and said: "Allah Almighty has made it lawful, and the Noble Prophet (peace and blessings be upon him) has permitted it."
It is narrated through another chain that ‘Umrah performed during the months of Hajj is also considered complete. The Noble Prophet (peace and blessings be upon him) acted upon this, and Allah has revealed its command in the Qur’an.
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 6392