Did the Prophet Muhammad say that the first Muslim to pray in Hagia Sophia would go straight to paradise?
Where can i find this? In the Quran, Hadith or ??

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

There is no verse in the Book of Allaah that indicates the meaning you mentioned, and we have not come across any narration in the reliable books that we have at hand that indicates that meaning. Also, we don’t think that there is a Saheeh [authentic] Hadeeth in this meaning because had such a matter been confirmed, it would have been famous in the books of the scholars. For more benefit, please refer to Fatwa 82172.

Allaah Knows best.