A quick search of Google netted this entry from Wikipedia. The page says that conflict started in 711 CE with Islamic expansion, specifically by the Umayyad Caliphate. While this doesn't qualify as a riot, this marks the start of violent relationships between the two groups. This is reiterated here and here, although the time frames differ slightly from each other.
If you want to know exactly when the first riots were (defined as "a noisy, violent public disorder caused by a group or crowd of persons, as by a crowd protesting against another group, a government policy, etc., in the streets" from Dictionary.com), that information may be impossible to find. There was almost certainly resistance against Muslim rule by Hindu groups, but this waned and waxed with the Muslim ruler. Some rulers were very open to Hindu culture, such as Akbar, and probably warranted little unrest from Hindus because he allowed them to maintain normalcy in day-to-day living. Other were more aggressive toward Hinduism, inciting revolts. The ones cited there are the late 17th century.
The complexity is compounded when you look at Hindu tolerance for diversity. Many Hindus probably felt that as long as Muslim rule didn't negatively impact the way they lived their lives, it didn't matter. Riots would be uncommon in an environment where people were already content.
In addition, the issue is further muddled by the large number of differing groups within India and those taking control of India, of which there were many. During these time periods there was no unified Hindu or Muslim group, but there may have been hatred between smaller sects. Unified deviance from one large group toward another would have been impractical given modes of transportation and speed of communication.
