Shaykhul Islaam Ibn Taymeeyah said: "al-'Ibaadah (worship) is obedience to Allaah by following that which He ordered upon the tongues of His Messengers." He also said: "al-'Ibaadah (worship) is a comprehensive term covering everything that Allaah loves and is pleased with - whether saying or actions, outward or inward." [Majmoo'ul Fataawaa (10/49)]

Imaam al-Aajurree said: The seeker of knowledge should know that Allaah - The Mighty and Majestic - has made the worship of Allaah obligatory upon him. And that this worship is not possible without knowledge, the seeking of which has also been made obligatory upon him. He should also realize that ignorance is not befitting for a believer. He should seek knowledge in order to dispel ignorance from himself, and to worship Allaah - The Mighty and Majestic - as Allaah has commanded, not as he desires to worship Him. So the seeker of knowledge should strive hard in his quest, be sincere in his striving, and not be amazed with himself. Rather he should recognize that this is a favour from Allaah upon him, since it was Allaah who gave him the ability to acquire such knowledge, by which he is able to fulfill his obligations and keep away from that which is forbidden. [Akhlaaqul Ulamaa (p. 43).]

Imaam ash-Shaafi'ee said: It is obligatory for every Muslim to learn the Arabic tongue, to the utmost of his ability in order to be able to profess through it that there is none worthy of worship other than Allaah, and that Muhammad is His slave and Messenger, and to recite in [the Arabic tongue] the Book of Allaah, and to speak in mentioning what is incumbent upon him - the Takbeer (of the prayer), and what [other matters] are commanded, the Tasbeeh, the Tashahhud, and others. [Ar-Risaalah (p. 93)]

'Abdullaah Ibn Mas'ood said: Indeed I hold that a person forgets knowledge he used to know because of the sins that he commits. [Related by Wakee' in Az-Zuhd (no. 329), and it's chain of narration is authentic.]

Wakee' said: I seek help in memorization by abandoning sin, and disobedience. [Related by Ibn Hibbaan in Rawdhatul 'Uqalaa (p. 39).]