Musnad Ahmad 519:2

Muhammad ibn Ubayd narrated to us, Dawud al-Awdi narrated to us, from Amir, from Haram ibn Khanbash, who said: I was sitting with the Messenger of Allah, may Allah bless him and grant him peace, and a woman came to him and said: "O Messenger of Allah, in which months should I perform Umrah?" He said: "Perform Umrah in Ramadan, for an Umrah in Ramadan is equivalent to a Hajj." *** Abdullah narrated to us, my father and Yahya ibn Ma'in narrated to me, they said: Waki' narrated to us, Sufyan narrated to us—and Waki' said once: and Sufyan said—from Bayan and Jabir, from al-Sha'bi, from Wahb ibn Khanbash al-Ta'i, who said: The Messenger of Allah, may Allah bless him and grant him peace, said: "An Umrah in Ramadan is equivalent to a Hajj."