Intellectual theft—copying whole portions of a book or a journal without citing their authors—is unlawful in Shari`ah. But the penalty of theft mentioned in the Qur’an is not to be applied to this kind of theft. Rather, it entails a disciplinary punishment that is up to the judge to decide.
As for paraphrasing other’s thoughts in a new form, this is not considered intellectual theft, but honesty requires citing the original author. Otherwise, it would be regarded as a kind of cheating which is forbidden, but does not entail a stated penalty or even a disciplinary punishment.
In this regard, we would like to cite for you the following fatwa issued by Al-Azhar Fatwa Committee:

The committee is of the opinion that quoting portion(s) of a book, a magazine, or any other piece of writing is lawful on the condition that the original authors of these writings are cited. Copying others’ writings and presenting them as one’s own thoughts is a kind of plagiarism that is unlawful both in the Shari`ah and in man-made laws.
As for using others’ thoughts by way of paraphrasing them and mingling them with one’s own thoughts, there is nothing wrong about that. This applies also to using religious and scientific opinions and theories, but citing the original th
inker or inventor in these cases is also a condition.
Intellectual theft, in Shari`ah, is not like stealing money or material property of others; this is to say, it does not require the penalty of theft prescribed in these cases. However, it may entail a disciplinary punishment, especially if it results in such negative effects as referred to in the question.

Moreover, Sheikh Muhammad Husain Fadl Allah, the well-known Shiite jurist, Lebanon, states:

First, it is not lawful for one to quote another’s piece of writing and present it as one’s own creation, for this is a kind of cheating and dishonesty. As for considering it a theft in Shari`ah, it follows in this regard the intellectual property rights stated in the general law or accepted norms. This is to say, he who plagiarizes another’s thoughts is to be regarded as committing a crime, just as he who attributes a book of another author to himself or prints the book for his own interest without taking the permission of the author.
Second, the legal ruling to be applied to a plagiarizer in this case is the punishment stated in the law for crimes of such a kind if it is proved that the accused person has used exactly another’s thoughts without this being a coincidence or without citing the source of these thoughts.

Dr. Zaki Badawi, dean of the Islamic faculty in Britain, further says:

Jurists have not dealt with intellectual theft. They have been, rather, concerned with tackling cases of stealing material properties whose penalty is mentioned in the Qur’anic verse: “As for the thief, both male and female, cut off their hands.” (Al-Ma’idah: 38)
The definition presented by jurists for theft in general does not apply to intellectual theft. Jurists defined theft as “a stealer’s seizing someone’s property in secret with the purpose of taking this property as his own.’ This is not the case with intellectual theft. On one hand, intellectual thoughts are not material property to be seized in secret; rather, they are plagiarized and presented in public. On the other hand, intellectual theft was confined to plagiarizing in poetry.
Besides, scholars used to quote from others’ writings without citing the source, for all would believe that knowledge is a common right for all people to the extent that some gave fatwas to the effect that teachers of the Qur’an should not receive returns for this task. This was because it was a custom in the past that thinkers or intellectuals not receive material rewards for their intellectual production.
This has changed today, as writers and intellectuals receive money for essays and books they produce. Hence, intellectual property has become parallel to material property, which requires, in turn, that it receives protection on part of Shar`iah.
Therefore, I see that he who commits a crime of plagiarism should receive a disciplinary punishment that is up to the judge to decide so that others may be deterred from committing similar crimes.

Dr. Fadel Al-Milani, a professor at the Islamic faculty, and a member of the council of Imam Al-Khaw’i’s institution in London, concludes:

In spite of the fact that the description of theft in Shari`ah does not apply to the question in hand, yet one’s copying others’ thoughts and attributing them to oneself is a clear dishonesty, and flagrant breaching of others’ rights to their intellectual properties.
Anyway, it is doubtless that dishonesty and breaching others’ rights are forbidden in Islam. Hence, I believe that if the plagiarizer benefits from stealing others’ thoughts and attributing them to himself, he is to compensate the original authors for his wronging them in this respect.