How to refuse your sponsor for the impossible deadline committed by him?

Thank you all for good answers. I would put this issue as a risk in my status report, and add the impact of this risk to the project (Scope to be cut down, increase the budge (resources)). If none of these are provided, then quality of the end product will be poor. 

Then, I will leave the decision for the Sponsor to take. Usually, in my experience we either de-scoped lower priority deliverables to next release, or added additional resources whenever such tighter deadlines we had...

Sometimes it is diffcult to achieve deadlines due to various factors (i.e. financial, manpower, technical, raw material etc.). In this situation, explain your sponsor about the diffculties to achieve deadline. Listen his/her counter questions carefully and give logical answers to satisfy him.

Nothings is impossible- be Self motivated

As my before comment: "nothing is impossible" but we have to take in consideration the technical aspects for each implementation phase.

In order to obtain your sponsor approval for possible implementation terms, you will have to analyse the facts, evaluate the efforts and the risks and give him the correct and logical arguments for renegotiation. 

just tell the sponsor straight in the eye that the deadline is impossible to meet by showing facts and schedules; convivce the sponsor else the project will just suffer

Drive directly to the point explaining  Why it is not possible . But if there are counter views proposed (which definitely would happen) , They shall be carefully analysed . The best way to be cautios is to minute the risks and  ensure the plan is executed at the maximum permissible efficiency. 

We have to communicate with the client regarding the deadline.