He said salaam before the end of the prayer by mistake then he started another prayer
Question

                I said salaam by mistake after the second rak’ah of a four-rak’ah prayer, then I said takbeer straight away for the Sunnah prayer. When I realized that I had not completed the obligatory prayer, at that moment I intended that the obligatory prayer that I had not completed and the naafil prayer that I did after it should be voluntary prayers, and I repeated the obligatory prayer and the Sunnah prayer. What is the solution in such cases if it should happen again in the future?.
Praise be to Allah.If a person forgets and 
    says the salaam before completing his prayer, then he remembers shortly 
    afterwards, then he should go back and complete his prayer and do the 
    prostration of forgetfulness, because of the report narrated by al-Bukhaari 
    (1227) and Muslim (573) from Abu Hurayrah (may Allaah be pleased with him) 
    who said: The Messenger of Allaah  (peace and blessings of Allaah be 
    upon him) prayed Zuhr or ‘Asr and he said the salaam (i.e. after two 
    rak’ahs). Dhu’l-Yadayn said to him: Has the prayer been shortened, O 
    Messenger of Allaah? The Prophet  (peace and blessings of Allaah be upon 
    him) said to his companions: “Is it true what he is saying?” They said: Yes. 
    He prayed two more rak’ahs then he prostrated twice. 
If he has started to offer 
    a naafil prayer, then he remembers that he did not complete the obligatory 
    prayer, he should stop the naafil prayer and complete the obligatory prayer, 
    then do the prostration of forgetfulness. 
Ibn Qudaamah said in 
    al-Mughni (1/373): The one who says the salaam before completing his 
    prayer by mistake, then realizes before a long time has passed or his wudoo’ 
    is invalidated, has to do what remains of his prayer, then recite the 
    tashahhud and say the salaam, then do the two prostrations of forgetfulness. 
    If a long time has passed or he has invalidated his wudoo’, then he should 
    repeat the prayer from the beginning. 
If he does not remember 
    until he has started another prayer, then it depends. If he has only done a 
    little of the second prayer and a long time has not passed, then he should 
    go back to the first prayer and complete it. If a long time has passed then 
    the first prayer is invalidated. This is the view of al-Shaafa’i. End 
    quote. 
In Kashshaaf al-Qinaa’ 
    (1/400) it says: If the one who said the salaam before completing the prayer 
    does not remember until he has started another prayer, he should stop it if 
    a short time has passed, and go back to the first prayer and complete it, so 
    that he can attain continuity between its essential parts, then he should do 
    the prostration of forgetfulness. End quote. 
There is no specific 
    definition of what counts as a long time or a short time; he should refer to 
    what is customary. 
As for making the first 
    (deficient) obligatory prayer a voluntary prayer, that is not correct, 
    because actions are by intentions, and you prayed it with the intention of 
    offering an obligatory prayer, and you exited it likewise. Because you 
    repeated the obligatory prayer after that, then you have done what was 
    required of you. But it would have been better for you to stop the naafil 
    prayer and go back to the obligatory prayer and complete it, then do the two 
    prostrations of forgetfulness after saying the salaam at the end of the 
    prayer, then pray the naafil prayer after that. 
And Allaah knows best.