Musannaf Ibn Abi Shayba 3:5818

Abu Bakr narrated to us, he said: Ibn ‘Ulayyah narrated to us, from ‘Ali ibn Zayd, from Abu Nadrah, from ‘Imran ibn Husayn, who said: I witnessed the conquest with the Messenger of Allah, may Allah bless him and grant him peace. He stayed in Makkah for eighteen nights, shortening the prayer and not praying except two rak'ahs. Then he would say to the people of the city: "Pray four, for we are travelers."