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.