Praise be to Allah.
There is no proof in the Prophet’s Sunnah -- as far as we know -- that it is mustahabb to raise the finger when saying the shahaadah after wudoo’, and it is well known that the basic principle with regard to acts of worship is to limit it to that which is proven by texts of the Qur’aan and saheeh Sunnah (tawqeef), and it is not permissible to add anything more than what is narrated in the Sunnah. What is prescribed for the Muslim to say after doing wudoo’ is, “Ashhadu an laa ilaaha ill-Allah wahdahu laa shareeka lah wa ashhadu anna Muhammadan ‘abduhu wa rasooluhu (I bear witness that there is no god except Allah alone with no partner or associate, and I bear witness that Muhammad is His slave and Messenger); it should not be limited to saying Laa ilaaha ill-Allaah only.
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) was asked: What is the ruling on raising the finger when saying the shahaadah following wudoo’, and doing it regularly?
I do not know of any basis for that. Rather what is prescribed for the one who finishes doing wudoo’ is to say: “Ashhadu an laa ilaaha ill-Allah wahdahu laa shareeka lah wa ashhadu anna Muhammadan ‘abduhu wa rasooluhu. Allaahumma aj’alni min al-tawwaabeen waj’alni min al-mutatahhireen (I bear witness that there is no god except Allah alone with no partner or associate, and I bear witness that Muhammad is His slave and Messenger. O Allaah, make me one of those who repent and make me one of those who purify themselves).”
Noor ‘ala al-Darb (Fataawa al-Tahaarah, Furood al-Wudoo’ wa Sifatuhu)
With regard to what the questioner mentioned of the words of Shaykh Ibn Jibreen (may Allah preserve him), we could not find any words of his to say that this action is mustahabb.
In the saheeh hadeeths there is mention of gesturing with the forefinger when saying the Tashahhud in prayer and when the khateeb says the du‘aa’ on the minbar on Friday. As for doing that after wudoo’, there is no proof for that.
Only the words of Allah and His Messenger (blessings and peace of Allah be upon him) may be described as decisive.
Based on that, it is not appropriate for us to say that the views of the scholars based on their understanding of the texts of the Qur’aan and Sunnah are decisive, or to ask, What is the decisive view concerning such-and-such? unless that view is based on definitive evidence from the Qur’aan and Sunnah, such as the prohibition on zina, the prohibition on drinking alcohol, etc.
But with regard to matters of ijtihaad, we cannot say, The decisive view is such-and-such; rather we should say: What appears to be the case is such-and-such, or The most likely to be correct is…. and similar phrases.
And Allah knows best.