Assalaamu alaykum. If someone fed ten poor people as an expiatation for having broken an oath but that person just guessed that those people were Muslims and did not confirm their religion because they live in a Muslim majority country, will it be valid? Or does the person have to distribute the food again?

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

If a person breaks his oath and he chooses to feed people as an expiation, then he must feed ten poor, free Muslims; Mukhtasar Al-Khiraqi reads, “If a person is obliged to expiate for breaking an oath, then he has the choice to feed ten poor, free Muslims, whether they are old or young, if they ate the food.”

Besides, Ibn Qudaamah says while speaking about the conditions of feeding people as an expiation, “The third condition: they must be Muslims, and it is not permissible to give the expiation to a non-Muslim, whether he is a Thimmi (a non-Muslim living under the care of the Muslim state) or belligerent.”

If a person who expiates for his oath is in a country where most of the population are Muslims and he acted according to what he had thought, that those to whom he gave the expiation were Muslims, then his expiation is sufficient, as certainty in such a matter is impossible, and what one predominantly thinks is enough in acts of worship.

Shaykh Ibn ‘Uthaymeen said in Ash-Sharh Al-Mumti’, “Predominantly thinking is enough in acts of worship, as the Prophet said, 'He should endeavor to find the truth and then act according to it.”

Shaykh Ibn ‘Uthaymeen also said in Majmoo’ Al-Fataawa, “Moreover, in many acts of worship, it is enough to act according to what one predominantly thinks, and certainty in these acts is not an obligation if it is impossible or hard to achieve.” 

Allah knows best.