al-Haafidh Ibn Rajab al-Hanbalee (رحمه الله) wrote in his treatise “Kashf-ul-Kurbah fee wasfi Haali Ahlil-Ghurbah” (English: Alleviating Grievances in Describing the Condition of the Strangers):And from this understanding, is what Abu Nu’aim and others have reported from Kumail Ibn Ziyaad  on the authority of ‘Alee, (رضي الله عنه), that he said:
الناس ثلاثة: عالم رباني، ومتعلم على سبيل نجاة، وهمج رعاع أتباع كل ناعق يميلون مع كل صائح، لم يستضيئوا بنور العلم، ولم يلجئوا إلى ركن وثيق“
The people are of three types: the learned scholar, the student of knowledge who is upon the path towards salvation and the uncivilized ignorant follower following every ideology, blowing in the direction of every wind. He is not guided by the light of knowledge nor is he established upon a firm pillar.”Then he mentioned some words on the benefit of knowledge, up to the point where he said:
هاه إن ههنا لعلما جما – وأشار بيده إلى صدره – لو أصبت له حمله“
Here! Indeed, here is immense knowledge – and he pointed to his chest – if only I could gather those who would carry it!
بل أصيب لَقِنا غير مأمون عليه مستعملا آلة الدين للدنيا، ومستظهرا بنعم الله على عباده وبحججه على أوليائه
But yes, instead I have found he who is unreliable and not to be entrusted with it. Doing the works of the Religion for the sake of this worldly life, making a demonstration of the favors of Allaah (i.e. the knowledge he was given) in front of His servants and using His arguments against His beloved ones (the true scholars).
أو منقادا لحملة الحق لا بصيرة في أحنائه، ينقدح الشك في قلبه لأول عارض من شبهة، ألا لا ذا ولا ذلك
Or he is one who is honest and obedient towards the carriers of truth, while possessing no understanding and insight (of his own) in his mind. Doubt has pierced his heart and troubled it upon the first time problems were presented to it. Indeed, he is neither this nor that.
أو منهوما باللذة سلس القيادة للشهوة، أو مغرما بالجمع شيء شبها بهما الأنعام السائمة، كذلك يموت العلم بموت حامليه
Or he is one who is greedy for fulfilling his pleasures. He has become submissive to the guidance of his desires or infatuated with the piling up of wealth and treasures. These individuals are not from among the shepherds of the Religion in any way. The closest things in resemblance to them are the cattle that graze about freely. Just like that, the knowledge has died with the death of its carriers. 
اللهم بلى لا تخلو الأرض من قائم لله بحجة إما ظاهرا مشهورا أو خائفا مغمورا لئلا تبطل حجج الله وبينات
O Allaah! Verily, do not isolate on this earth, he who establishes the evidences for Your sake, whether he be apparent and well known or fearful and obscure in order that the evidences and clear proofs of Allaah may not be refuted.
وكم ذا وأين أولئك؟ والله الأقلون عددا والأعظمون عند الله قدرا، يحفظ الله بهم حججه وبيناته حتى يودعوها نظراءهم ويزرعوها في قلوب أشباههم
And how many of them are there? Where are they? I swear by Allaah, they are the fewest in number, yet the greatest in stature in the sight of Allaah. Allaah preserves His evidences and manifest proofs through them until their likes desist from it and plant it into the hearts of those similar to them.
هَجَم بهم العلم على حقيقة البصيرة، وباشروا روح اليقين واستلانوا ما استوعره المترفون، وأنسوا بما أستوحش منه الجاهلون
Through them, knowledge is fortified upon the reality of evidences. And they spread the glad tidings of the spirit of certainty. They soften what those who give in to excessiveness have roughened. And they draw close to that which the ignorant feel repelled by and estranged from.
وصحبوا الدنيا بأبدان أرواحها متعلقة بالمحل الأعلى، أولئك خلفاء الله في أرضه والدعاة إلى دينه، آه آه شوقا إلى رؤيتهم. انصرف إذا شئت
They accompany the world with their bodies while their souls are drawn towards the great abode. They are Allaah’s aides on His earth and the callers towards His Religion. And they long for their look (at Him in Paradise).”
End of ‘Alee, (رضي الله عنه)’s words.
[Note: This report has been transmitted by Abu Nu’aim in al-Hilyah (1/79) and al-Khateeb in al-Faqeeh wal- Mutafaqqih (1/49). Also see Jaami’ Bayaan-ul-‘Ilm of Ibn ‘Abd-il-Barr (2/112) and al-Bidaayah wan- Nihaayah of Ibn Katheer (9/47). Shaikh Saleem Al-Hilaalee mentioned it in one of his articles in al-Asaalah magazine (Sha’abaan issue no. 3 of 1413H).]
[Source: Kashf-ul-Kurbah fee wasfi Haali Ahlil-Ghurbah, page 323, written by the great Imaam, al-Haafidh Zayn-ud-Deen Ibn Rajab al-Hanbalee (رحمه الله).]
 He is Kumail Ibn Ziyaad An-Nakha’ee (رحمه الله). He was one of the righteous and noble amongst the Taabi’een. He was captured and killed by Al-Hajaaj Ibn Yoosuf Ath-Thaqafee in 82H. See Al-Ma’rifah watTaareekh (2/481) of Al-Fasawee (رحمه الله).
 The great scholar Ibn Abeel-’Izz Al-Hanafee (رحمه الله) said in his book al-Itibaa’ (pg. 86):“In this narration, the Commander of the Believers has divided the blameworthy carriers of knowledge into three categories:
1) The wicked innovator who is not trustworthy and has no Eemaan. He is the one who rejects the truth which the Book came with, despises the creation and argues about the verses of Allaah without any authority. There is only pride in his heart,
2) The blind-follower that follows without any clear guidance or certainty, and
3) The one who follows the desires of the body and wealth.”