Whenever someone uses the term "steal" with regards to scientific contributions, be wary. Even with regards to actual espionage (the Soviet Union did not "steal" or even "copy" the American atomic bomb, despite using espionage information, as I have written about at length), much less transference of knowledge. (Did the US "steal" European scientific knowledge in the mid-20th century to become the chief scientific superpower in the world? Did the Byzantines "steal" the knowledge of Ancient Greece and Rome when they preserved it for centuries, so it could be then used by others? Did Apple "steal" the many US-government-produced technologies that it combined into the iPhone? This is a ridiculous verb to use.)

The Islamic Golden Age did use the intellectual contributions of the Ancient Mediterranean World as well as those from India (and the Indians in turn used and innovated on knowledge from further east as well, etc.). They also innovated on this knowledge: they were not slavish imitators (as is sometimes alleged post-Rome Western European knowledge producers were), nor mere preservationists (the latter is a common criticism of the Byzantine Empire), but actively engaged in the business of scientific criticism and knowledge growth. They produced some outstanding works that of course built on what had been worked on elsewhere (what else ought scientists do?), but created important works of synthesis and new arguments and observations that broke dramatically with the past (their medical knowledge, while informed by Greek and Roman commentators, also went in dramatically different directions in some areas; they didn't just, for example, imitate or copy Aristotle and Galen, but actively critiqued and engaged with these texts, adding their own observations, theories, approaches).

Furthermore, the impact of the Islamic Golden Age work on Western European science was immense, though they frequently Latinized the Arabic names in a way that can hide their origins. Avicenna (Ibn Sīnā), Alhazen (al-Haytham), Algorithmi (al-Khwarizmi) — it is impossible to understand the development of Western European science, medicine, and mathematics without the key works by these Islamic Golden Age scholars. There is a good reason that so many fundamental concepts in mathematics, astronomy, and medicine in particular carry names that are derived from Arabic (algebra, alcohol, algorithm, alkali, alchemy... every time you see an "al-" prefix, it's probably got an Arabic connection, and there are plenty of other words that came by way of Arabic).

Ask yourself why someone talking about scientific contributions, ones with immense and clear impact on the further development of science globally, would denigrate a thriving and important culture that had the insight to collect, preserve, and build-upon the insights of other civilizations prior to them as "stealing." It's an absurd slur, obviously anti-Islamic, and probably also a thinly-veiled racism. One wonders what word they would use to describe the Renaissance Europeans, who proudly resurrected Ancient texts, usually translated from Arabic, in their effort to retrieve past glory and then, later, build upon it. I suspect they don't use the term "steal."

One should always beware of what the intellectual historian David Hollinger refers to as the bigot-booster problem: when you highlight some culture or people for making exceptional contributions, you run the risk of people flipping it to its exact opposite for whatever reason (he coined the term this up in the context of attitudes towards Jews, but it applies well here, too). The Islamic Golden Age is wrongly overlooked in European-centric histories of science (which is why many people find it so interesting and "surprising" in the West, because they have been taught nothing about it), but it was but one period in the larger development of scientific inquiry. It was not the be-all and end-all. For a few centuries, the Islamic world was the center of immense knowledge production and use. For still-debated reasons, that eventually ended. There are many ways to interpret this history, especially from the perspective of the modern world. But to dismiss any of this as "stealing" is, well, ridiculous. The idea that they would have needed to be totally independent of other sources of information, both temporally and geographically, to be granted respect is, well... at best it is a silly notion of the history of science. No historian of science of any merit would suggest such a thing.

There is so much wrong with the initial supposition that it's hard to know where to start. For a recent, short article that gives a far better understanding of how scholars in the medieval Islamic world thought about the knowledge of other places and times, see Scott Montgomery, "Mobilities of Science: The Era of Translation into Arabic," Isis 109, no. 2 (2018), 313-319.