With regards to the effects of sins upon the ability of the student of knowledge to memorise, ‘Abdus Salaam ibn Burjis (d. 1425 AH) rahimahullaah said:
Disobeying Allaah, the Most High, is an obstacle to attaining Sharee’ah knowledge – because it (Sharee’ah knowledge) is a noor (light) from Allaah which He casts into the hearts of whomever He wishes from His servants.
And noor (light) and dhulmah (darkness) do not come together in a heart.
Therefore ibn Mas’ood radi Allaahu ‘anhu said:
Indeed I consider that a man (might) forget knowledge which he has already learned because of a sin which he does.
And may Allaah have mercy upon ash Shaafi’ee who said:
I complained to Wakee’ about my memorisation being bad
So he directed me to leave off sins
And he informed me that knowledge is a noor (light)
And the light from Allaah is not bestowed upon a person who is sinful
Quoted in ‘Awaa-iq ut talab of ‘Abdus Salaam ibn Burjiss rahimahullaah page 38, translated by Nasser ibn Najam
‘Alee radi Allaahu ‘anhu said:
Knowledge calls out for action, so either it is responded to or else it departs.
And Sufyaan ath Thawree rahimahullaah said:
Knowledge is only to be learned in order for Allaah to be feared and obeyed by it.
And Fudayl ibn ‘Iyaad rahimahullaah said:
The person of knowledge does not stop being jaahil (ignorant) of what he knows until he acts on it. So when he acts on it, he is an ‘aalim (person of knowledge).
And ibn al Jawzee rahimahullaah said:
The true miskeen (impoverished person) is the one whose lifetime is lost in knowledge that he does not act upon. So he has missed out on the delights of the dunyaa and the treasures of the Hereafter.
Thus he will come (on the Last Day) as a bankrupt person, along with having a strong proof against himself.
Quoted in ‘Awaa-iq ut talab of ‘Abdus Salaam ibn Burjiss rahimahullaah pages 17 to 19, translated by Nasser ibn Najam
The noble scholar Muhammad ibn Saalih al ‘Uthaymeen rahimahullaah said, addressing students at the Imam Muhammad ibn Sa’ood Islamic University at al Qaseem on 25th Jumaada al Oolaa, 1417AH:
At this moment you are in this place at (your) differing levels – from them the high (levels) and from them lesser than that.
However regardless of where the person reaches in knowledge, then he is (still) in need of knowledge – as per the statement of Allaah, the Blessed and Most High:
But over all those endowed with knowledge is the All Knowing One (Allaah).
Soorah Yoosuf (12) aayah 76
Therefore I urge you to have a general eagerness in pursuing (your) studies; and that you know with certain knowledge that the person who does not strive with all his efforts in attaining knowledge, then knowledge in its entirety will slip away from him.
And therefore it is said: indeed knowledge is the most miserly thing there is! For if you give it your all, you will attain (only) some of it; and if you give it only some (of your all), then it will elude you entirely.
And therefore I urge you, O young people, to have a complete eagerness for knowledge.
(Quoted in Wasaayaa wa tawjeehaat li tullaab il ‘ilm pp 139 to 140, translated by Nasser ibn Najam)
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)