Shaykh Saalih al-Fawzaan
Source: Things that Nullify One's Islam

One of the kinds of apostasy is mockery of what Allah sent down or of anything that the Messenger came with, even things from the Sunnahs (of the Prophet, sallallaahu 'alayhe wa sallam) or recommended things like siwaak, trimming the moustache, removing armpit hair, or trimming the nails. If a person makes mockery of any of these things he becomes a disbeliever. The evidence for this is found in the Statement of Allah the Most High:
"And if you were to ask them, they would say: 'Surely we were only jesting and playing.' Say: 'Was it Allah, His Verses,
or His Messenger you were mocking? Make no mistake, verily you have disbelieved after your belief.' "
So then the one who mocks anything that the Messenger came with, be it obligatory or recommended, he is an apostate from the Religion of Islam. So then what do you think about the one who says, "Growing the beard, trimming the moustache, removing armpit hair, washing the knuckles, these are insignificant things!" This is exactly what is meant by mockery of the Religion of Allah, the Mighty and Majestic. When they say this thing, and if they know (it is from the Religion), then they have apostated, since this is belittling what the Messenger (sallallaahu 'alayhe wa sallam) came with. It is obligatory to have great reverence for the Sunnah of the Messenger (sallallaahu 'alayhe wa sallam) and to respect it.
Even if a person falls into some kind of opposition (to the Sunnah) due to his desires, then surely he still must respect the way of the Messenger (sallallaahu 'alayhe wa sallam), respect his Sunnahs, and respect the narrations. He must not say, "These are insignificant things."

The proof is the Statement of Allah the Most High:
"Say: 'Was it Allah, His Verses, or His Messenger you were mocking? Make no mistake, verily you have disbelieved after your belief.' "
The event that led to the revelation of this Verse was what happened with a group of people who were with the Messenger (sallallaahu 'alayhe wa sallam) during the Battle of Tabook. They were Muslims. During a gathering they began to say, "We have not seen the likes these reciters of ours, the most untruthful tongues, the greediest stomachs, the most cowardly in the face of the enemy..." They were referring to the Messenger of Allah (sallallaahu 'alayhe wa sallam) and his Companions.
Amongst them was a young man from the Companions who became enraged at this speech and went to convey what this group was saying to the Messenger of Allah (sallallaahu 'alayhe wa sallam). He found that the revelation had preceded him (in informing the Messenger).
So then the group came apologizing once they heard that the Messenger had become aware of what happened in their gathering. One of them was hanging on to the front harness of the Prophet's she-camel while he (sallallaahu 'alayhe wa sallam) was riding it, saying, "O Messenger of Allah! We were just talking loosely to help pass the time during the journey! We did not intend mockery, rather we only intended to joke." The Messenger (sallallaahu 'alayhe wa sallam) would not even look at him, he only recited upon him this Verse:
"And if you were to ask them, they would say: 'Surely we were only jesting and playing.' Say: 'Was it Allah, His Verses,
or His Messenger you were mocking? Make no mistake, verily you have disbelieved after your belief.' "
Notice His Statement, "Verily you have disbelieved after your belief." This proves that before they had uttered words (of mockery) they were Muslims, and that when they uttered them they apostated from Islam, even though they were saying that it was a joke. This is because the affairs of the Religion are not to be played with, so Allah had declared them to be disbelievers after their faith. We ask Allah for safety. [4]
This is also proof that whoever insults Allah, His Messenger, His Book, anything from the Quran, or anything from the Sunnah of the Messenger (sallallaahu 'alayhe wa sallam) apostates from Islam, even if he was joking.
Where are those who say that he does not apostate unless he intends it in his heart? They say, "If a person insulted Allah, the Messenger, or the Quran, we do not pass a judgement upon based merely on his utterance or his action." Where do they get these statements and these specifications from? Allah has judged them with apostasy while they were saying:
"We were jesting and playing."
They were believers in Allah and His Messenger, people of tawheed, however, once they uttered those words, Allah said:
"Verily you have disbelieved after your belief."
And He did not say, "If you had truly believed that." We ask Allah for safety.
So it is obligatory that things are put in their proper places and that we do not add things, take them away, or specify the texts from our own selves. Allah did not ask about what they believed, nor did He mention that they believed (what they said), rather He ruled on them with apostasy after having faith:
"Verily you have disbelieved after your belief."
He based this (ruling) on a statement. He based this (ruling) on mockery, and He did not specify it with these specifications. If a person speaks with a word of disbelief without being forced into it, then he is judged with apostasy. However, if he was forced, then he has not apostated in this case.