Musnad Abu Ya'la al-Mawsili 110:127

Harith ibn Surayj narrated to us, Mu'tamir narrated to us, Layth narrated to us, from Abu Fazara, from Yazid ibn al-Asamm, from Ibn Abbas, who said: The Messenger of Allah (peace be upon him) said: "I have not been commanded to construct high/sturdy mosques." Ibn Abbas said: "You will surely decorate them just as the Jews and the Christians decorated [theirs]."