Genesis 30:38, 41

And he set the rods which he had peeled over against the flocks in the gutters in the watering-troughs where the flocks came to drink; and they conceived when they came to drink.

And it came to pass, whensoever the stronger of the flock did conceive, that Jacob laid the rods before the eyes of the flock in the gutters, that they might conceive among the rods