What are the islamic sources that relate about India or Hinduism?

Surely the thousands of Nabi's and Rassul's that appeared before Muhammad (saws) were not preaching the exact same precepts as Islam, at least because they were sent to totally different people. Of all of these messengers and prophets, some must have been in India. There is a well-known hadith that means "go seek for knowledge wherever it is, even in China". What are the Islamic sources that relate to India or Hinduism in any other way than to treat them as polytheists? Did Muhammad (saws) ever say anything about this? Thank you.