How do I map EC2 instance to committed usage discount purchase??

Do you mean you want to "lock" the commitment discount to a particular running instance? Or do you need some reporting, for example to check where exactly the discount applies?

You definitely cannot "lock" the discount to a particular resource. I understand you purchased Savings Plan. SPs are designed in such a way that the discount will be applying to your matching running resources automatically. This is to ensure that resources that will benefit from the SP the most will be covered first (the resources that will provide you highest discount % will be covered first).

In terms of "looking back" to check where exactly the discount applies, you could use Cost Explorer (to an extent), and you could also use AWS Cost and Usage Reports - https://docs.aws.amazon.com/cur/latest/userguide/use-cases.html

For simpler reporting, you can check a few things in Cost Explorer:

• you can apply filter under "charge type" and find value that will be something like "savings plan covered usage"; you can then switch between "unblended cost view" and "amortized cost view" to see how the discount applies across the whole term of your commitment; though, you can't see the resource IDs (actual instances) unless you enable hourly and resource level data in Cost Explorer (which may cost extra)
• you can also use RI/SP coverage and utilization reports to monitor additional information, for example, how much you're actually saving over time (in the RI or SP utilization report), or how much of your usage or resources are actually covered by your commitment discounts