Those who spoke on performing Umrah outside the months of Hajj

Ata was asked: If someone performs Umrah in a month other than the months of Hajj, is a hady obligatory upon him? He said: No hady is obligatory upon him, though the Companions used to offer hady; and in the year the polytheists stopped the Prophet (peace and blessings be upon him), he (peace and blessings be upon him) also offered a sacrifice. (The narrator asked:) Had he (peace and blessings be upon him) entered ihram for Umrah? He said: Yes, and peace was made with those polytheists on the condition that they would come the following year.

And I have seen Sayyiduna Mu'awiyah (may Allah be pleased with him) slaughter a camel for an Umrah performed outside the months of Hajj.