הלכה על ערכין 64:61

From the laws of the commandment is that which they, may their memory be blessed, said (Sifra, Behar, Section 1:2) that the obligation of the commandment of this count is only after the conquest and division of the land, as it is stated (Leviticus 25:3), "Six years shall you sow your field and six years shall you prune your vineyard" - until each and every one recognizes his land. And from when the tribe of Reuven and Gad and half [of the] tribe of Menashe were exiled, this commandment became negated, because the Jubilees were negated from that time and onward; as it is sated (Leviticus 25:10), "and you shall proclaim freedom in the land for all of its inhabitants" - only when all of its inhabitants are upon it; and also that they not be mixed up (see Arakhin 32b), but rather sitting in their proper order. And at the time when the Jubilee is practiced in the Land [of Israel], it is [also] practiced outside of the Land; as it is stated (Leviticus 25:11), "It is a Jubilee" - meaning in every place (Kiddushin 38b). And at the time when the Jubilee is practiced, the law of the Hebrew slave is practiced, as well as the law of the houses of a walled city, the law of a consecrated (cherem) field and the field of a holding; and we accept a resident stranger (ger toshav).