Musnad Abu Ya'la al-Mawsili 110:13

Uthman ibn Abi Shaybah narrated to us, Ibn Abi Za'idah narrated to us, from Ibn Abi Layla, from 'Ata', from Ibn Abbas, who said: The Prophet, may Allah bless him and grant him peace, did not perform any 'Umrah except in the month of Dhu al-Qa'dah.