How does an atheist pray while being an infidel, and will he be rewarded for his prayers during his period of infidelity if he repents?

Whispers concerning creed are treated by seeking Allah's help and through supplication, especially during prostration. As for one whose heart has opened to disbelief, he is invited to Islam by clarifying the proofs of Allah's Oneness and the truthfulness of Prophet Muhammad, peace and blessings be upon him.

Regarding the rewarding of a disbeliever, after embracing Islam, for his previous acts of worship, there is a difference of opinion among scholars: Those who prohibit it interpret the hadith "You embraced Islam upon the good you had previously done" in various ways, among them that this refers to acquiring beautiful characteristics beneficial in Islam, or beautiful praise, or an increase in his future good deeds. Those who permit it understand the hadith literally and cite the hadith "When a disbeliever embraces Islam and his Islam is good, Allah Almighty records for him every good deed he previously performed." They explained the jurists' saying "No worship from a disbeliever is valid" as referring to worldly rulings, not the reward in the hereafter.