What is the ruling on the difference in determining the first day of Ramadan fasting between two mosques, one of which follows Saudi Arabia and the other the European Council for Fatwa and Research?

If the European Islamic Association relies on theing of the moon, then it is better to follow them in fasting and breaking fast. The issue of differing moon-sighting locations is an old and permissible disagreement, but the more preponderant view is that each country has its own sighting, and not everyone is obliged to follow the sighting of another country. This is indicated by the hadith of Kuraib with Ibn Abbas, may Allah be pleased with them both, where Ibn Abbas did not adhere to the moon-sighting of Syria, and he reported that the Prophet, peace and blessings be upon him, commanded them with that.