Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) said: Its companion (Umrah's pairing with Hajj) is present in the Book of Allah: "And complete the Hajj and the Umrah for Allah" [Surah al-Baqarah: 196].
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 8764