Sahih Al Bukhari – Book of Fasting

Volumn 003, Book 031, Hadith Number 168. 
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Narated By Anas bin Malik RA : We used to travel with the Prophet PBUH  and neither did the fasting persons criticize those who were not fasting, nor did those who were not fasting criticize the fasting ones

Lesson: if matters are flexible in religion then don't complain or criticize.

However

 you should have clear understanding of the matter if it is flexible. please never try to add new things in religion.