Ramban

AND HE BROUGHT HIM FORTH OUTSIDE. According to the simple interpretation of Scripture it means that He brought him forth from his tent into the open so that he could see the stars. And according to the Midrash its explanation is as follows: G-d said to him, “Leave your astrological speculations, for you have seen by the constellations that you are not destined to raise a son. “Abram” indeed may not have a son, but “Abraham” will have a son; “Sarai” will not bear a child, but “Sarah” will bear a child. Thus the language of Rashi. But ‘Abram’ did have Ishmael! The meaning of the Midrash, however, is that Abram sought a son who would qualify to become his heir, [which excluded Ishmael, who would not inherit him], even as he said, And, lo, my household slave will be mine heir. The Holy One, blessed be He, then said to him, “This man shall not be thine heir, but one born of thine own body shall be thine heir, and leave your astrological speculations. ‘Abram’ will not have a son as his heir, but ‘Abraham’ will have a son as his heir.” It is also possible that the astrological speculation concerned the pair together, namely, the “Abram and Sarai” as a pair will not beget children, and the Eternal now assured him that “Abraham and Sarah” will beget children. In my opinion, however, [the Divine assurance now given to Abraham did not mention Sarah, rather], her name is an addition on the part of the Midrash, meaning that such indeed was the case also with Sarah. The Holy One, blessed be He, however, did not assure him concerning Sarah at the present time. Even at the time of the prophecy concerning the circumcision, Abraham was still in doubt whether “Sarah” would bear a child.