Ubaydullah reports from Ibn Abbas (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) stayed fifteen days in the year of the Conquest and shortened the prayer.
Irak bin Malik narrated it from the Prophet (peace and blessings be upon him) mursal, and the narration of Ikrimah from Ibn Abbas is more authentic than all of that; and Allah knows best.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5470