Fiqh-us-Sunnah Volume 2, Supererogatory Prayer, Fiqh 2.075.

Abu Musa al-Ash’ari and ‘Umar ibn ‘Abdulaziz prayed in a church. Ash-Sh’abiy, ‘Ata, and Ibn Sireen did not see anything wrong with praying in a church [if one happened to be in a church at the time of salah]. Al-Bukhari says: “Ibn ‘Abbas would pray in churches [under unusual circumstances] except for those with statues or sculptures.” The Muslims of Najran wrote to ‘Umar saying that they found no place cleaner or better to pray in than a church. ‘Umar wrote to them: “Sprinkle it with water and leaves and pray therein.” According to the Hanafi and Shaf’i schools, it is disliked to pray in such places in general.