Prayer: Confidence Regarding Separation Before Four Farsakhs When Travelling Under Another's Authority
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 1272
With regard to someone who is travelling under the authority of someone else, if he is not confident that he will become separated from the other person before reaching four farsakhs and that he will not travel any further, he must perform tamām prayers. However, if he is confident about this, he must perform qaṣr prayers.
Third condition: one must not change his intention on the way [i.e. he must not decide against travelling]. If before travelling four farsakhs one changes his mind [about travelling] or becomes unsure [about travelling], and the distance he has already travelled together with the return journey totals less than eight farsakhs, he must perform tamām prayers.