Umm Ma’qil narrated that the Prophet said: “Umrah during Ramadan is equal to Hajj.” Graded Sahih by Al-Albani (Jami` at-Tirmidhi 939).
Ibn ‘Abbas (May Allah be pleased with them) reported: The Prophet (ﷺ) said, “(The performance of) ‘Umrah during Ramadan is equal to Hajj (pilgrimage).” Or said, “Equal to the performance of Hajj with me.” [Al-Bukhari and Muslim]
“Equal to the performance of Hajj” means the return and reward to which Hajj is eligible. It does not mean that it will serve as a substitute for Hajj and absolve one from the need to perform it. This distinction of `Umrah in the month of Ramadan is perhaps for the reason, that in this way two acts of worship are combined at one time.