It was narrated that Ibn ‘Abbas said:

The Messenger of Allah (ﷺ) went on a journey and stayed for nineteen days, offering the prayers two [rak"ahs] by two. Ibn "Abbas said: When we travelled and stayed for nineteen days, we offered the prayers two [rak"ahs] by two, but if we stayed longer than that, we offered the prayers with four [rak"ahs].