It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that once the Prophet (peace and blessings be upon him) set out on a journey, and he stayed there for 19 days but continued to perform the prayers as two rak‘ahs each. Therefore, when we also travel and stay for 19 days, we perform the prayers as two rak‘ahs, that is, shortened. And when we stay for more than that, we perform four rak‘ahs, that is, the full prayer.