Is it mustahabb to use the siwaak when entering the mosque?

Some scholars said that, but the correct view is that it is not mustahabb to use the siwaak when entering the mosque, because that is not proven from the Prophet (blessings and peace of Allaah be upon him). If that were prescribed, the Messenger (blessings and peace of Allaah be upon him) would have done that and would have encouraged his ummah to do that. 

Following the Prophet (blessings and peace of Allaah be upon him) means doing what he did, and refraining from what he refrained from. Just as we do not refrain from doing what the Prophet (blessings and peace of Allaah be upon him) did of acts of worship, by the same token we do not do that which he refrained from. 

Shaykh Muhammad ibn ‘Uthaymeen (may Allaah have mercy on him) said: 

We have an important basic principle which is: just as doing some things is Sunnah, refraining from doing some other things when there is a reason for doing them is also Sunnah, even though it is refraining and is not doing. An example of that is whether it is Sunnah or not to use the siwaak when entering the mosque. 

Some of the scholars say that it is Sunnah to use the siwaak when entering the mosque, and they based that on the fact that when the Prophet (blessings and peace of Allaah be upon him) entered his house, he would begin with the siwaak. So they drew an analogy between entering the mosque and entering the house, and they said if a person uses the siwaak when entering his house, so that he can meet his family with a clean mouth, then when he enters the mosque he should do it so that he may converse with his Lord with a clean mouth. But we say, if the Prophet (blessings and peace of Allaah be upon him) used to enter the mosque and it is not narrated that he would start with siwaak when he entered the mosque, and if it were Sunnah the Prophet (blessings and peace of Allaah be upon him) would have done it, then the Sunnah is not to use the siwaak when entering the mosque, based on the idea that the reason for using siwaak would be entering the mosque. But if when he enters the mosque he is going to pray two rak’ahs immediately, and he wants to use siwaak because of the prayer, and not because of entering the mosque, then this is prescribed. End quote. 

Al-Sharh al-Mumti’ (4/362).