Wash and soak both the rice for atleast four hours.
Wash and soak the urad dal also for atleast four hours.
Drain and mix.
Grind to a fine batter adding water as required.
Transfer the batter in a deep vessel, cover with a tight fitting lid and keep it in a dark and warm place overnight to ferment.
Just before making the dosas, add salt to taste and mix lightly. Heat a tawa.
Drizzle a little oil and wipe with a wet cloth so that the tawa is at the right temperature.
Drop a ladle full of batter on the tawa and spread it in a circular motion as thin as possible.
Drizzle a little oil all around and let the dosa cook on medium heat.
When ready the edges will begin to separate from the tawa.
Gently roll into a cylinder and serve hot with masala, chutney and sambhar