Ikrimah (may Allah have mercy on him) narrates from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) "stayed nineteen days in the year of the Conquest and shortened the prayer, that is, he kept praying two rak'ahs each."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5466