مُسْنَدُ أَحْمَدَ
Musnad Ahmad 1957
Musnad of 'Abdullah bin al-'Abbas bin 'Abdul-Muttalib from the Prophet
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].
Musnad Ahmad 1958Sahih (Darussalam) [ al-Bukhari (1080)]