Abu Hurairah (Radhiallaho anhu) reported:
The Messenger of Allah (Sallallaahu ‘alaihi wasallam) said,
“(The performance of) `Umrah is an expiation for the sins committed between it and the previous `Umrah; and the reward of Hajj Mabrur (i.e., one accepted) is nothing but Jannah.”
[Al-Bukhari and Muslim].
This Hadith tells us that the merit of `Umrah is that it is an expiation for sins. But this, too, is an expiation for minor sins. `Umrah includes assuming Ihram, circumambulation of Ka`bah, going between As-Safa and Al-Marwah and Halq or Taqsir (shaving of head or cutting hair). Whether or not `Umrah is an obligation like Hajj is arguable among the `Ulama’. Some people, like Imam As-Shafi`i, hold that it is obligatory; others like the followers of the Hanafi school of thought regard it Sunnah Mu`akkadah; still others think it to be voluntary. Imam Al-Bukhari is inclined to the first view which is also endorsed by sayings of some Companions of the Prophet (Sallallaahu ‘alaihi wasallam) about which some Muhaddithun (scholars of Hadith) have even claimed that these sayings are Mausul, the chain of narrators reaches up to the Prophet (Sallallaahu ‘alaihi wasallam). (For further details, please see Fath Al-Bari.) Otherwise, the view that it is non-obligatory is preferable.
Riyad-us-Saliheen – Hadith No. 1275