Musnad Ahmad 317:3

Sufyan ibn 'Uyaynah narrated to us, he said: Ibn al-Munkadir narrated to us, he said: I heard Yusuf ibn 'Abd Allah ibn Salam saying: The Messenger of Allah (peace and blessings of Allah be upon him) said to a man from the Ansar and his wife, when they performed 'Umrah in Ramadan: "For an 'Umrah in Ramadan is for you both like a Hajj." And Sufyan said once: And he did not say "narrated to me," meaning Ibn al-Munkadir: "For an 'Umrah in it is like a Hajj."