Praise be to Allah.
If a Muslim is in a country that relies on moonsighting to determine the beginning and end of the month, then he is enjoined to start and end the fast when they do. We have stated that in the answer to question no. 12660.
But if the Muslim is in a kaafir land, or in a land where they tinker with the beginning and end of the month according to their whims and desires, and do not pay attention to the moonsighting as prescribed in Islam, then there is no sin on him if he follows those whose moonsighting and adherence to Islamic rulings he trusts. See the answer to question no. 50522, where this is explained further.
And Allaah knows best.