And he brought him forth abroad, and said, Look now toward heaven, and tell the stars, if thou be able to number them: and he said unto him, So shall thy seed be. (Genesis 15:5)
This shows that the vision was given to Abram at night. Still in vision, the patriarch was led out into the open and bidden to lift his eyes to the starry sky and count—if he could—its myriad glittering orbs, if he would know the number of his seed.