Muhammad bin Khazim al-Darir محمد بن خازم أبو معاوية الضرير · The Book on Traveling

"The Messenger of Allah traveled on a journey and he prayed two Rak'ah for nineteen days." Ibn Abbas said: "So when we would stay somewhere for nineteen (days) we would pray two Rak'ah , and if we stayed longer than that we would complete the Salat."