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

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.