With regards to the student of knowledge acting upon his knowledge, Shaykh Muhammad ibn Saalih al ‘Uthaymeen rahimahullaah comments:
Therefore it is a must that (he) act upon what he has knowledge of – because if he does not act upon his knowledge he becomes from the first of those who will be burned by the Fire on the Day of Resurrection:
The scholar who does not act upon his knowledge
Will be punished before the worshippers of idols
This is one aspect – if he does not act upon his knowledge, it will result in disappointment regarding the knowledge, and no barakah (blessing), and nisyaan (forgetfulness) of the knowledge… And therefore it was reported from Imaam ‘Alee radi Allaahu ‘anhu that he said: Knowledge calls out for action, so either it is responded to or else it departs…
So what is it that departs? Knowledge, and this is something clear – because if you act upon (your) knowledge, you will remember it each time you act.
And I will strike an example for you – that of a man who knows the description of the salaat (prayer) from the Sunnah. So he begins to act upon it whenever he prays. Will he forget what he has learned? He will not forget because he keeps on repeating it. However, if he were to leave off acting upon it, he would forget. And this is an evidence which can be perceived (by us) – that acting upon knowledge necessitates firmness upon (that) knowledge, and that (the person) does not forget it.
(Sharh hilyah taalib il ‘ilm p24-25 of Shaykh Muhammad ibn Saalih al ‘Uthaymeen rahimahullaah, translated by Nasser ibn Najam)