Praise be to Allaah.  

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. 

From the fatwas of Shaykh ‘Abd al-‘Azeez Aal al-Shaykh. Majallat al-Buhooth al-Islamiyyah, 61/82.