"To me You have given no offspring." (Gen 15:3)
Abraham did not pray for children, even though Sarah was barren.
This was because he did not believe himself to be worthy of a miracle. Another explanation is that he did not wish to take his reward in This World, and so he left it up to the Holy One to do as He wished.
If you contend that he declared, "To me You have given no offspring…"; this was not a prayer, but rather like a person declaring a fact.
[Zohar I, 137b]
He took him outside. (Gen 15:5)
The Sages taught: Children, health and livelihood do not depend upon one's merits and righteousness, but on one's mazal.
He thus raised Abraham above the stars….
"Mazal" does not mean "luck", as is usually understood. Rather it refers to the sefira of keter from which divine energy flows forth (in Hebrew, "nozel", directly related to the word "mazal") to the other sefirot, and ultimately to the physical world. The practical implications of this outflow can be foretold using the science of astrology (as known to the Sages), which examines the position of the heavenly constellations (mazalot) at the time of a person's birth.
We deduce that Abraham saw in his constellation that he would not have a son. But the Holy One "took him outside", which the Rabbis interpret as meaning: "Dissociate yourself from your astrological predictions." He thus raised Abraham above the stars [i.e. beyond the realm of reason and nature] and told him, "Now gaze at the heavens and count the stars…," promising him that his offspring would be as numerous as the stars.
We must now explain these words of the Rabbis: Before the Torah was given to Israel all creatures were dependent upon their mazal, including children, health and livelihood. But when G‑d gave the Torah to Israel, He removed the control of the stars and constellations over them [for the Torah itself transcends the world].
Anyone who studies Torah… nullifies the power and influence of the constellations….
This we learn from Abraham, for his descendants would in the future receive the [Hebrew letter] hei from his name, alluding to the five books of the Torah.
Originally, Abraham was called "Abram". Later the letter hei was added to his name making "Abraham". The numerical value of the letter hei equals five.
Regarding this the verse states, "These are the products of the heaven and the earth when they were created [in Hebrew, 'behibaram']" (Gen. 2:4)
"Abraham" has the identical letters as "behibaram".
He said to Abraham: Because of the hei which had been added to your name the heavens will be in your control and all the stars and constellations which give forth light will be subjugated to you!" Accordingly, anyone who studies Torah in order to fulfill the commandments nullifies the power and influence of the constellations over himself. If he does not study Torah [in order to fulfill the commandments] it is as if he did not study at all, and he remains subject to the influence of the starts and constellations.
[Zohar III, 216b]
[Translation and commentary by Moshe Miller
First published by Fiftieth Gate Publications and Seminars]