Amr bin Hazm (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) wrote a letter to the people of Yemen containing the obligatory duties (fara'id), the sunnahs and the rulings on blood-money (diyat), and he sent that letter with me, and in it was this: "Umrah is the lesser Hajj."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 8771