Praise be to Allah.
The scholars are agreed that it is mustahabb to use the siwaak when doing wudoo’, because the Prophet (blessings and peace of Allah be upon him) said: “Were it not that it would cause hardship for my ummah, I would have instructed them to use the siwaak every time they do wudoo’.”
Narrated by al-Bukhaari in a mu‘allaq report. Also narrated by Ibn Khuzaymah in his Saheeh (140) and classed as saheeh by al-Albaani in Irwa’ al-Ghaleel, 1/109
It was also narrated by Ahmad (9612): “Were it not that it would cause hardship for my ummah, I would have instructed them to use the siwaak with every wudoo’.”
Classed as saheeh by al-Albaani in Saheeh al-Jaami‘, 5317.
The scholars differed concerning the time for using the siwaak when doing wudoo’. Some of them said that it should be before saying Bismillah for doing wudoo’ and some of them said that it should be when rinsing the mouth.
Ibn Nujaym (may Allah have mercy on him) said:
There was a difference of opinion concerning the time for that. In al-Nihaayah and Fath al-Qadeer [which are books of the Hanafi madhhab] it says that it should be when rinsing the mouth. In al-Badaa’i‘ and al-Mujtaba it says that it should be before doing wudoo. But most scholars are of the former view, which is more likely to be correct, because it makes the mouth cleaner.
End quote from al-Bahr al-Raa’iq, 1/21
Al-Zarkashi (may Allah have mercy on him) said:
The fact that it is mustahabb to use the siwaak is confirmed in several places, at the time of prayer and when rinsing the mouth during wudoo’.
End quote from Sharh al-Zarkashi, 1/30
Al-Shirwaani (may Allah have mercy on him) said:
The view of the author – meaning Ibn Hajar al-Haytami – is in accordance with the majority, that it should be before saying Bismillah. And the view we adopt is that it should be after washing the hands and before rinsing the mouth.
End quote from Hawaashi al-Shirwaani, 1/221
What seems to be the case is that the matter is broad in scope, because there is no clear evidence from the Sunnah to stipulate any particular time.
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said:
Using the siwaak when doing wudoo’ should be with rinsing the mouth, because this is the point at which the mouth is purified and the siwaak is for purifying the mouth, as it is narrated in a saheeh report from the Prophet (blessings and peace of Allah be upon him) that he said: “The siwaak is purifying for the mouth and pleasing to the Lord.” So the siwaak should come with rinsing the mouth, but if you wish you may use the siwaak after finishing wudoo’ or if you wish you may use it before you start, but it is better if it is done when rinsing the mouth.
End quote from al-Sharh al-Mukhtasar ‘ala Buloogh al-Maraam, 2/44
He also said:
The scholars said that it should be used when rinsing the mouth, because rinsing the mouth is purifying the mouth, so it should be done at that point. But if that is not possible, then it may be done after wudoo’. The matter is broad in scope.
End quote from Liqa’ al-Baab al-Maftooh, 133/31
The apparent meaning of the words of Shaykh al-Albaani (may Allah have mercy on him) indicates that he thinks that the siwaak should be used before saying Bismillah for wudoo’, as he said:
The way it – wudoo’ – is done is: using the siwaak, saying Bismillah, washing the hands three times – which is Sunnah – rinsing the mouth, snuffing water into the nose and blowing it out.
End quote from al-Thamar al-Mustataab, p. 9
And Allah knows best.