(a) Ikrimah bin Khalid said: I asked Ibn Umar (may Allah be pleased with them both) about performing Umrah before Hajj, and he said: There is no harm if someone performs Umrah before Hajj.
(b) Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) said: the Prophet (peace and blessings be upon him) performed Umrah before Hajj. Imam al-Bukhari (may Allah have mercy on him) recorded it in his Sahih from the narration of Ibn al-Mubarak and Abu Asim from Ibn Jurayj.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 8736