Musnad Ahmad 2:80

Al-Walid ibn Muslim narrated to us, Al-Awza'i narrated to us, that Yahya ibn Abi Kathir narrated to him, from Ikrimah, the freed slave of Ibn Abbas, who said: I heard Ibn Abbas saying: I heard Umar ibn al-Khattab saying: I heard the Messenger of Allah, peace and blessings of Allah be upon him, while he was in al-Aqiq, saying: "An emissary from my Lord came to me tonight and said: 'Pray in this blessed valley, and say: An Umrah within a Hajj.'" Al-Walid said: Meaning, Dhu al-Hulayfah.