I'm assuming you meant to say "water" not "warmer" in the question. ADH has nothing to do with warmth, so it was probably a typo.
Antidiuretic hormone, or ADH, is produced in the hypothalamus of the brain and stored just below the hypothalamus in the posterior pituitary gland. It is released when the osmolarity of blood rises (there is less water than other substances in the blood). Once it reaches the kidney, it targets the distal and collecting ducts and changes them so that the epithelium is more permeable to water. This causes the reabsorption of water and increases the concentration of urine, reduces urine volume, and lowers blood osmolarity to its set point. At this point, a negative feedback mechanism reduces the secretion of ADH.