Sardar says nothing more than wikipedia. He quotes the same passage and claims that it stands on its own. He italicizes the last phrase. If al-Majriti emphasized it in the original, I would take that as his making the claim.
He says "compare this experiment with that of Lavoisier." Indeed, Lavoisier also used mercury, but it is terrible for this experiment, as I explained above, and maybe it caused Lavoisier to fool himself, too. But if that had been Lavoisier's last experiment, he wouldn't have convinced anyone. Who cares if one burn conserves mass? The point of the law is that if properly contained, all burns conserve mass. If al-Majriti weighed the container, that would be (1) closer to Lavoisier and (2) expressing an interest in the correct law, but the recorded weight sounds like the reagent, not the container.
How do you judge the independence of sources? I can certainly find you others that agree with Sardar.
Added: I suppose you could look for agreement between sources with different politics.
Here is a Holmyard book from 1945 that quotes the same passage, but does not believe the author is proposing the law. Incidentally, he disputes the attribution to al-Majriti. (In 1922 in Nature, Holmyard announced that he had a new manuscript and gave the translation of that passage that he copied in 1945 and which wikipedia copied; Sardar gives a new translation.)
The metaphysical principle of the conservation of matter—that matter can be neither created nor destroyed in chemical processes—called upon here is at least as old as Aristotle (Weisheipl, 1963).
-Michael Weisberg, Paul Needham, and Robin Hendry, Stanford Encyclopedia of Philosophy
The citation seems to refer to this:
Weisheipl, James A. (1963), “The Concept of Matter in Fourteenth Century Science”, in Ernan McMullin, ed., The Concept of Matter in Greek and Medieval Philosophy, Notre Dame: University of Notre Dame Press.
I wasn't able to obtain a jail-broken copy of the paper; maybe you'll have better luck.
One of the sharpest and most important tools in the LessWrong cognitive toolkit is the idea of going meta, also called seeking whence or jumping out of the system, all terms crafted by Douglas Hofstadter. Though popularized by Hofstadter and repeatedly emphasized by Eliezer in posts like "Lost Purposes" and "Taboo Your Words", Wikipedia indicates that similar ideas have been around in philosophy since at least Anaximander in the form of the Principle of Sufficient Reason (PSR). I think it'd be only appropriate to seek whence this idea of seeking whence, taking a history of ideas perspective. I'd also like analyses of where the theme shows up and why it's appealing and so on, since again it seems pretty important to LessWrong epistemology. Topics that I'd like to see discussed are: