If his prayer is invalidated or interrupted, should he say the tasleem?
Question

                If the prayer becomes invalidated or the worshipper interrupts a naafil prayer to join the imam in a prayer, should he say the tasleem to leave the prayer, or what should he do?.
Praise be to Allah.
    If something happens to the worshipper whilst he is praying 
    that means that he has to stop the prayer – such as if he starts a naafil 
    prayer then the iqaamah for prayer is given – then the intention to stop the 
    prayer is sufficient, and he should not say the tasleem, because the proper 
    place for the tasleem is at the end of the prayer. ‘Ali ibn Abi Taalib (may 
    Allaah be pleased with him) said: The Prophet  (peace and blessings of 
    Alalah be upon him) said: “The key to prayer is purification, one enters it 
    by saying takbeer (‘Allaahu akbar – Allaah is Most Great)’ and one exits it 
    by saying tasleem (‘Al-salaamu aleikum’ – peace be upon you’).” Narrated by 
    the authors of Sunan apart from al-Nasaa’i, with a saheeh isnaad. 

    With regard to the one whose prayer becomes invalidated, he should stop his prayer without saying tasleem and with no intention, because his prayer has been invalidated.