Prayer: Changing the Intention to Stay During the First Four Rakʿah Prayer
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 1331
If a traveller who has made the intention to stay somewhere for ten days changes his mind during his first four rakʿah prayer within its prescribed time, in the event that he has not yet started to perform the third rakʿah, he must complete his prayer as a two rakʿah prayer and perform his remaining prayers in qaṣr form. If he has started to perform the third rakʿah but has not yet gone into rukūʿ, he must sit down and complete the prayer in qaṣr form. However, if he has gone into rukūʿ, he can either break his prayer or complete it, and he must perform it again in qaṣr form.