There are two ways to get the datasource value in your case.

1. You need to provide datasource item from your layout where you are calling the Rendering as partial as given below:


        @{
            var datasourceItem = "Guid or Path of item"
        }

        @Html.Sitecore().Rendering("RenderingPath", new { DataSource = datasourceItem })

Or if your data-source will always remain same then you need to provide/enter the datasource value into Datasource field of rendering and then by the below code you can get the value of datasource field on rendering.

    RenderingContext.CurrentOrNull.Rendering.RenderingItem.DataSource


Hope this will help you.