Related for Chullin 209:58

Washing [hands] before [the meal] (Mayim Rishonim) is optional. [Washing hands] after [the meal] (Mayim Acharonim) is obligatory. [When he washes] Mayim Rishonim, if he wants to interrupt [the water flow over his hands and pour the water over them the second time] he may do so. [However, when he washes] Mayim Acharonim, if he wants to interrupt [the water flow over his hands and pour the water over them the second time] he may not do so, [but rather he must pour the whole cup of water over the hands in one shot].