مسجد قباء
September 622 CE (Rabi' I, 1 AH): at the Hijrah, while the qibla still faced Jerusalem
On reaching Quba at the Hijrah, the Prophet ﷺ stayed with Banu 'Amr b. 'Awf (four nights in Ibn Ishaq, ten-odd in Bukhari) and founded this mosque, the first built in Islam, on a ground where Kulthum b. al-Hidm dried his dates. He marked out the qibla and laid the first stone himself; Abu Bakr and 'Umar laid theirs beside it, and the rest followed along the line. The stones came from the harra, the black lava field. This model shows the mosque at that founding, when prayer still faced Jerusalem: a low wall of dry-laid stone round an open court, and a shelter of palm fronds over the place of prayer. Sixteen or seventeen months later the qibla turned to Makkah and the Prophet ﷺ founded the mosque again facing it; that and every later rebuilding stood on the same site. No source gives the first mosque's size, the height of its walls or its doors, so those are estimates, as are the sizes and places of the houses and towers round about.
Sources: al-Samhudi, Wafa' al-Wafa bi-Akhbar Dar al-Mustafa (Beirut: Dar al-Kutub al-'Ilmiyya) 1:191–197, 3:16–28 · Ibn Sa'd, al-Tabaqat al-Kubra 1:180, 1:188 · Ibn Hisham, al-Sira al-Nabawiyya 1:493–494 · Sahih al-Bukhari 3906 (the stay at Quba and the founding), 403 and 4491 (the qibla change at Quba) · Qur'an 9:108 (a mosque founded on taqwa from the first day) · First qibla from Quba: azimuth to Jerusalem ≈ 333° (computed)