Hamza Yusuf in Buddha in the Qur'an (viewed at Scribd.com) writes:
In classical Muslim literature on religions and sects, we find many references to "al-Badadah," meaning the Buddhists, as well as "al-Budd," the Buddha himself. Ibn al-Nadim (d. 998), an Iraqi bookseller and author of the famous work entitled The Compendium (al-Fihrist), who catalogued existing authors and their subjects of study, records books that deal with Buddhism, including The Life of Buddha (Kitab al-Budd). In his chapter entitled "Notes on the Buddha," Ibn al-Nadim delineates the different scholarly views of the Buddha: some believed he was the divine incarnate, while others claimed he was a messenger of God; still others thought Buddha to be a generic name for those who guided others onto the right path. ...
This makes me curious as to which Islamic scholars claimed that Buddha was a messenger of God. The paper continues:
Perhaps the most significant classical Muslim description of Buddhism is found in Imam Muhammad b. 'Abd al-Karim al-Shahrastani's (d. 1153) comprehensive survey entitled, Religions and Sects (al-Milal wa al-nihal). ... In this work, he also makes a rather stunning--and intriguing--statement connecting the Buddha to a character in the Qur'an. ...
Based upon their description of the Buddha, if they are accurate, it would seem that he is none other than al-Khadir, whom Muslims acknowledge, upon him be peace.
The phrasing suggests that al-Shahrastani did not hold this belief true, but rather acknowledged it as a possibility. Therefore, I'm not sure who Hamza Yusuf is referring to.
Question: Which scholars claimed Buddha was a messenger of God?