Musnad Ahmad 2:248

Hammad al-Khayyat narrated to us, Abdullah narrated to us, from Nafi': That Umar added to the mosque from the pillar to the enclosure, and Uthman added [to it]. And Umar said: "Had I not heard the Messenger of Allah, peace and blessings of Allah be upon him, say: 'We intend to increase [the size of] our mosque,' I would not have added to it."