Fiqh-us-Sunnah - 5.121

Hadith 5.121.1

In Arabic the word 'Umrah is derived from al-i'timar which means a visit. Here it means paying a visit to Ka'bah, performing lawaf around it, walking between Safa and Marwah seven times, and then shaving one's head or cutting one's hair short.
There is consensus among scholars that it is a prescribed worship. Ibn 'Abbas reported that the Prophet (peace be upon him) said, "An 'Umrah in the month of Ramadan is equal (in reward) to performing a Hajj.'' (This does not, however, remove from one the obligation of performing the obligatory Hajj) This was reported by Ahmad and Ibn Majah.
Abu Hurairah reported that the Prophet (peace be upon him) said, "From one 'Umrah to the next is an atonement for any sins committed in between, and the reward for an accepted Hajj is nothing but Paradise." This was reported by Ahmad, Bukhari and Muslim.

Hadith 5.121.A

Repeated Performance of 'Umrah

Nafi' said, "During the rein of Ibn Az-Zubair, Abdullah bin 'Umar performed an 'Umrah twice a year." Al-Qasim said, "Aishah performed three 'Umrahs in one year."
Some people asked if any of the scholars criticised her for this. He replied, "May Allah be blessed! (Criticise) the Mother of Believers?!!" The majority of scholars supports this view. Malik, however, holds that the repetition of 'Umrah more than once a year is disliked.

Hadith 5.121.B

Umrah Before and During the Months of Hajj

A person may perform 'Umrah during the months of Hajj, without performing Hajj. 'Umar performed 'Umrah during the month of Shawwal and returned to Madinah without performing Hajj. One may, like 'Umar, perform 'Umrah even before Hajj.
Taw'us said, "In the Days of Ignorance the Arabs considered performing an 'Umrah during the months of Hajj as one of the major sins. And they used to say, "When (the month of) Safar ends, and the camels are back, and wounds in their legs are healed, and when traces of footprints (of pilgrims) are effaced, then one wishing to perform 'Umrah may do so."
After the advent of Islam, the people were commanded to perform 'Umrah even during the months of Hajj. Thus, the 'Umrah was permitted during the months of Hajj till the Day of Judgement.