This source claims that there is a Chovas Halevavos that quotes the Islamic Hadith without attribution. I was wondering if there are other examples of this in Jewish texts.
I do not have any specific citations but according to Rabbi Lamm in Torah uMadda:
In the Geonic period, according to the eleventh-century R. Joseph Ibn Aknin (in his commentary to the Song of Songs), R. Hai Gaon (939-1038), 'the last and greatest of the Geonim,' did not hesitate to use Arabic sources, including Arabic love songs, to prove a Talmudic point; in addition, he quotes the Koran and Hadith (the sayings and doings of Muhammed). (page 22).
In rabbinic sources after the rise of Islam? Certainly, there was a cross-pollination of ideas. It may not be that a given rabbi was reading the Hadith per se, it may have been that they heard something from someone and said "wow, that's powerful." They may have known it was recorded as Hadith; or that it was loosely based on Islamic thought; or not at all. The "non-attribution" thing isn't such a big deal; there were points in time when the scholarly style was "I will say my thing, and you can figure out for yourself what sources this is drawing on." (Rabbi Pelkovitz points this out about the Sforno.)
Here's a blog on the quotation found in both Chovot HaLevavot (and then later rabbinic sources) and the Hadith about a victorious army returning home -- "you've won the easy war; now comes the hard war [i.e. how to live in peacetime with justice and a sense of purpose]." He quotes material about the noted overlap between Chovot HaLevavot and the Hadith.
Similarly, there have been rabbis who were unimpressed by scholars who could quote a great number of sources but couldn't think critically about them. The description used was -- "it's like a donkey carrying a giant load of books." It's said this colorful put-down also originated in Islamic thought.
Look, the Talmud says (Sanhedrin 38b) that the sage Rabbi Meir (who lived around the year 100) would teach Proverbs with all sorts of stories about foxes. Doesn't that sound like somebody else? That's okay!