Prayer: Intending to Stay Based on the Supposed Intention of Friends
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 1337
If someone makes an intention to stay somewhere for ten days because he thinks his friends want to stay there for ten days, and after performing a four rakʿah prayer within its prescribed time he realises that they did not have such an intention, then even if he changes his mind to stay there he must perform tamām prayers as long as he stays there.