I know that there is a Gemarah (somewhere in Shabbos I think), that discuses birth signs, based on days of the week and month. The Gemarah ends with Ein Mazal B'Yisroel and a few stories that illustrate this point. Does anyone know exactly where it is?
And [Hashem] took [Avraham] outside and said, "Gaze, now, to the heavens, and count the stars if you can count them!" And He said to him, "So shall your offspring be!"
Bereishis Rabbah (44:12) has a problem. Avraham wasn't inside (or rather, there's no indication that he was), so what does it mean that Hashem took him outside (as explained by Maharzu to the Midrash)? It thus gives three answers, the latter of which mean that we would no longer be under the influence of the constellations:
R' Yehudah said in the name of R' Yochanan: "He raised him above the dome of the sky; this is what He said to him, 'Gaze, now, to the heavens,' as 'gazing' only means looking down from above" (as in Yeshaya 63:15 and Tehillim 80:15 - "Gaze down from the heavens and see" - Maharzu).
The Sages said: "'You are a prophet, not an astrologer,' as the verse says, 'But now, return [Avraham]'s wife, for he is a prophet [and he will pray on your behalf and you will live' (Bereishis 20:7; a prophet, not an astrologer, understands that prayer can change things - Maharzu, Eitz Yosef).
R' Levi said: While the sandal is on your foot, walk on the thorn. All who are placed below them should fear them, but you, who are placed above them, step on them.
This Midrash doesn't use the phrase "There is no Mazel in Yisrael;" however, it is the basis for the phrase.
In Nedarim 32a, Avraham asks Hashem what will be, as he is destined not to have children; Hashem responds that he should ignore his astrology, as there is no Mazel in Yisrael. This discussion parallels ours in Bereishis 15, except that it's referring not to our pesukim but those regarding Avraham's Bris in Bereishis 17. However, in Shabbos 156a, R' Yehudah says in the name of Rav this same discussion in our passuk, using the same derashah as the Midrash. Also in that Gemara in Shabbos, R' Yochanan learns it from a passuk in Yirmiyahu (10:2):
Do not be disheartened by portents in the sky; let the nations be disheartened by them. The nations, but not Yisrael.
It should be noted that R' Chanina in that Gemara in Shabbos holds that there is Mazel for Yisrael, but that it wisens and richens.