Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) said: "While I was at al-Aqiq, Sayyiduna Jibril (peace be upon him) came to me and said: Pray two rak'ahs in this blessed valley, and say: 'Umratun fi hajjah' (an Umrah within a Hajj). For Umrah has indeed entered into Hajj until the Day of Resurrection."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 8847