is a blameworthy innovation (bidʿah); but whoever leaves Mecca intending to perform an ʿumrah (1), then if (2) he enters one of those mosques and prays there for the sake of assuming ihrām, there is no objection to that.
In the time of the Prophet—may Allah bless him and grant him peace—and his Rightly Guided Caliphs, no one ever left Mecca to perform an ʿumrah (3) except for a valid excuse (4), whether in Ramadan or (5) at any other time. Among all those who performed Hajj with the Prophet—peace and blessings be upon him—there was not a single person (6) who carried out an ʿumrah from Mecca after the Hajj except ʿĀʾishah, as already mentioned, and even this was not practiced by the Rightly Guided Caliphs.
As for the Companions who preferred the isolated performance (ifrād), they merely recommended that one undertake Hajj in one journey and ʿumrah in another; they did not (7) deem it recommended to perform Hajj and then immediately follow it up (8) with a Meccan ʿumrah (9). In fact, they never did so at all—save for a (10) truly exceptional case, if it ever occurred.