Hadith 955
Sayyiduna Ibn Abbas (may Allah be pleased with them both) narrates that the Messenger of Allah (peace and blessings be upon him) undertook a journey, and he stayed for nineteen days and continued to perform prayers as two rak‘ahs. Sayyiduna Ibn Abbas (may Allah be pleased with them both) says: Therefore, we also perform two rak‘ahs (shortened prayer) during travel for nineteen days, and if we stay for more than that, then we perform four rak‘ahs (complete prayer).