"Vr. 16:43. (1191) 'Zikr' is one of the names of the Holy Qur'an as well as the Holy Prophet Muhammad (15:9). According to the interpretation of the Holy Ahlul-Bait, 'Zikr' here means the Holy Prophet and by his 'Ahlaz-Zikr'. The 'Ahlul-Bait' [are] the divinely chosen ones of the family of the Holy Prophet), [thus] the Imam of every age is the 'Ahle-Zikr' of the age (UB., MFA., DM., Ibne Murdwaih). In the very next verse, the Holy Qur'an is called 'Zikr'. Thus, the 'Ahlaz-Zikr' could be no other than the Holy Prophet Muhammad and his 'Ahlul-Bait'.

Ar. 'Zikr' literally means to have something present in the mind, [i.e.,] to be conscious of an object as opposed to [be] sub-conscious and unconscious. It has been used figuratively in any stimulus which brings object in the focus of consciousness. In the Holy Qur'an, it is always used to mean to be conscious of God—mindful of God—or the stimulus which makes one conscious of God. In this sense, the Holy Qur'an and the other scriptures and the Holy Prophet Muhammad are termed as 'Zikr'.

...'Zikr' here either means to be conscious of God and 'Ahl' means those who are always conscious of God as asserted in Vr. 24:37 or 'Zikr' here means the Holy Qur'an or the Holy Prophet. 'Ahl' will mean the people identified with the Holy Prophet and the Holy Qur'an, and they are those purified by God (33:33) and are those who are always with the Holy Qur'an and Holy Qur'an is with them. Refer to Hadithu Thaqalain.
 

(Source: S. V. Mir Ahmed Ali's translation of the Holy Qu'ran, pp: 856-857)

“Kumayl says that after this brief dissertation, Imam Ali pointed towards his chest and said,
 
“Look Kumayl! Here I hold stores and treasures of knowledge. I wish I could find somebody to share it with me. Yes, I found a few, but one of them, though quite intelligent, was untrustworthy, he would sell his salvation to get hold of the world and its pleasures, he would make religion a pretense to grasp worldly power and wealth, he would make this Blessing of Allah (knowledge) serve him to get supremacy and control over friends of Allah and he would through knowledge exploit and suppress other human beings. The other person was such that he apparently obeyed truth and knowledge, yet his mind had not achieved the true light of religion, at the slightest ambiguity or doubt he would get suspicious of truth, mistrust religion and would rush towards skepticism. So neither of them was capable of acquiring the superior knowledge that I can impart.

But this earth will never be without those persons who will prove the universality of truth as disclosed by Allah, they may be well-known persons, openly and fearlessly declaring the things revealed to them or they may, under fear of harm, injury or deaths hide themselves from the public gaze and may carry on their mission privately so that the reasons proving the reality of truth as preached by religion and as demonstrated by His Prophet may not totally disappear. How many are they and where could they be found?

I swear by Allah that they are very few in number but their worth and their ranks before Allah are very high. Through them Allah preserves His Guidance so that they, while departing, may hand over these truths to persons like themselves. The knowledge which they have acquired has made them see the realities and visualize the truth and has instilled into them the spirit of faith and trust. The duties which were decreed as hard and unbearable by them. They feel happy in the company and association of things which frighten the ignorant and uneducated. They live in this world like everybody else but their souls soar to the heights of Divine Eminence. They are media of Allah on this earth and they invite people towards Him. How I love to meet them!
 
O Kumayl ! I have told you all that I have to say, you can go back to your place whenever you like.””
 

(Source: Nahjul Balagha: Sermons, letters and sayings of Hazrat Ali, Jafery translation (1981), Saying no. 146, p.290)