Ikrimah (may Allah have mercy on him) reports from Ibn Abbas (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) went on a journey and stayed nineteen days, during which he (peace and blessings be upon him) kept praying two rak'ahs each. And Ibn Abbas (may Allah be pleased with them both) said: When we travelled and stayed nineteen days, we would pray two rak'ahs each; when we stayed longer than that, we would pray four rak'ahs.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5463