Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) said: Whoever performs Umrah in the months of Hajj, that is, Shawwal, Dhu al-Qa'dah and Dhu al-Hijjah, has become a mutamatti' (one performing tamattu'), and a sacrifice is obligatory on him; if he does not find one, he should fast.
In the previous chapter we have mentioned the statement of Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) concerning tamattu', that it is not complete unless he offers a sacrifice; if he has no sacrificial animal, he should fast: three fasts during Hajj and seven when he returns home. An Umrah outside the months of Hajj is complete without a sacrifice or fasting.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 8892