What Azure assembly has reference to ServiceBusTriggerAttribute?

You need to include the Microsoft.Azure.WebJobs.ServiceBus NuGet package as mentioned in the Prereq section of this article https://azure.microsoft.com/en-us/documentation/articles/websites-dotnet-webjobs-sdk-service-bus/#prerequisites

The Microsoft.Azure.WebJobs.ServiceBus package from Nuget contains the ServiceBusTriggerAttribute class.

This answer is correct for Functions 1.x, but if you are using Functions 2.x, then you need to install Microsoft.Azure.WebJobs.Extensions.ServiceBus:

Install-Package Microsoft.Azure.WebJobs.Extensions.ServiceBus

Here is the Microsoft documentation Azure Service Bus bindings for Azure Functions.

Also, make sure you reference the latest package and update any dependent packages.