Hadith.731 - Saleh ibn Al-Hakam reported that Imam Jafar ibn Muhammad Al-Sadiq (as) was asked about praying in synagogues and churches.
Imam (as) said: "Pray in them."
Salih then asked: "Even if they (non-Muslims) are praying there?"
Imam (as) said: "Yes. Do you not recite the Qur'an: 'Say, each acts according to their manner, but your Lord (azj) knows best who is rightly guided' (Surah Al-Isra, 17:84)?
Pray facing the Qibla and leave them (to their practices)."