Many people ask why Muslims don't start the fast Ramadan and celebrate their 'Id's on the same day. And even if many madhhabs claim that this is the chosen way, but it seems to me hardly practical without modern communication tools such as phone and internet or the imposition of a rule/ruler. (see also What are the opinions on moonsigthing as per the classical scholars?)
I'd like to know whether there is any evidence in Muslim sources where we find that all Muslims started fasting or celebrated their 'Id on the same day after the early days of Islam (and before our modern days and the invention of phones and the internet) where the spreading the news of the moonsighting would be possible within a few hours and before the next day started (after the time of the Prophet and his first Caliph).
If such an evidence exists it would be great to know how they could realize this?