In large saucepan, place chicken, water, onion, celery and salt. Bring to a boil, cover and simmer on low heat 1 hour. Remove skin and cut chicken into bite-size pieces. Strain liquid and use to cook rice according to package directions. In large frypan, place 1/4 cup of the butter over medium heat. Add mushrooms and sauté. Stir in soup and sour cream. Add rice, chicken, pimento and water chestnuts. Place in casserole; sprinkle with almonds and remaining butter. Bake in 350°F. oven 1 hour.