Jabir bin Abdullah (may Allah be pleased with him) said: A man asked the Messenger of Allah (peace and blessings be upon him): Is Umrah obligatory? He (peace and blessings be upon him) said: "No, but if you perform Umrah, it is better for you."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 8752