Making fun of the beard or clothes whose length etc. is in accordance with the Sunnah or other aspects of the Sunnah makes a person a kaafir, if he knows that this was proven from the Prophet (peace and blessings of Allaah be upon him), because he is thereby making fun of the words and actions of the Prophet (peace and blessings of Allaah be upon him). In this case he is opposing the Prophet (peace and blessings of Allaah be upon him) and making fun of his Sunnah, and the one who makes fun of the Sunnah and knowingly mocks things that are proven to be part of the Sunnah is not a Muslim.
Allaah says (interpretation of the meaning):
“Say: ‘Was it at Allaah, and His Ayaat (proofs, evidences, verses, lessons, signs, revelations, etc.) and His Messenger that you were mocking?’
Make no excuse; you disbelieved after you had believed”
If a person is called to observe one of the rulings of Islam and says that piety is in the heart but he does not follow the shar’i ruling, he is an evil liar. For faith includes both words and deeds, it is not simply the matter of what is in the heart. This view is the same as that of the evil innovators of the Murji’ah who restricted faith to the heart and said that it had nothing to do with outward actions. Moreover, if the heart is sound and filled with faith, that will be manifested in a person’s actions. The Prophet (peace and blessings of Allaah be upon him) said: “In the body there is a piece of flesh which, if it is sound, the entire body will be sound, and if it is corrupt, the entire body will be corrupt. Indeed it is the heart.” (Narrated by al-Bukhaari, 52; Muslim, 1599). And he also said: “Allaah does not look at the outward appearance or wealth of any one of you; rather he looks at your hearts and deeds.” (Narrated by Muslim, 2564)
Whatever the case, these words of those who stubbornly refuse to follow the truth and carry out the commands of Islam is a sign of their lack of faith by means of which they want to stop those who seek to call them to Islam and advise them.
Sheikh Muhammed Salih Al-Munajjid