("And they shall bring the heifer down) to a hard river-bed": And whence is it derived that it is valid even if it is not hard? From (Ibid.) "in the river bed" — even if it is not hard.
("a hard river-bed) which shall not be worked": I might think that it is forbidden to comb flax there and to chisel stones there; it is, therefore, written "and which shall not be sowed." Sowing was included in all (labor). Why was it singled out? To make it a paradigm, viz.: Just as sowing is distinctly in the soil, so, all (such labors are forbidden), to exclude those which are not labors of the soil.