Da'as Zekenim (in Pasuk 23), Hadar Zekenim (in Pasuk 25), Rosh and Moshav Zekenim (in Pasuk 33), citing the Tanchuma: The first time, it stopped in the field, with room on both sides ? if he wants to curse the descendents of Avraham, he may do so from one side (Yishmael) or the other (B'nei Keturah), but not in the middle (Yitzchak). It next stopped in Mish'ol ha'Keramim, which has room to go two abreast. If he wants to curse B?nei Yitzchak, he may do so only from one side (Eisav), but not the other (Ya'akov). The third time there was no room on either side; he may not curse any descendents of Ya'akov, for they are all Tzadikim. Rosh ? "be'Makom Tzar" hints at Ya'akov ? as the Torah writes in (Vayshlach Bereishis, 32:8) "va'Yeitzer lo".