Here is the passage, which I simply paraphrased yesterday. It's on page 124 of Stambaugh's translation of Being and Time. It's hard to drop the needle into the middle of Heidegger, so to speak, but those of you who know enough will be able to figure it out:
In which direction must we look for the phenomenal characteristics of being-in as such? We get the answer to this question what we were charged with keeping in view phenomenologically when we pointed out this phenomenon: being-in in contradistinction to the objectively present insideness of something objectively present “in” an other; being-in not as an attribute of an objectively present subject effected or even initiated by the objective presence of the “world”; rather, being-in essentially as the kind of being of this being itself. But then what else presents itself with this phenomenon other than the objectively present commercium between an objectively present subject and an objectively present object? This interpretation would come closer to the phenomenal content if it stated that Da-sein is the being of this “between.” Nonetheless, the orientation toward the “between” would still be misleading. It colludes unawares with the ontologically indefinite approach that there are beings between which this being as such “is.” The between is already understood as the result of the convenientia of two objectively present things. But this kind of approach always already splits the phenomenon beforehand, and there is no prospect of ever again putting it back together from the fragments. Not only do we lack the “cement,” even the “schema,” according to which this joining together is to be accomplished has been split apart, or never as yet unveiled. What is ontologically decisive is to avoid splitting the phenomenon beforehand, that is, to secure its positive phenomenal content.