Prayer: Performing Qaṣr Prayers When Obligated to Perform Tamam Prayers
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 1348
If someone who must perform tamām prayers performs them in qaṣr form, his prayers are invalid in all circumstances. And based on obligatory precaution, this rule also applies to a traveller who has an intention of staying somewhere for ten days and performs qaṣr prayers there on account of not knowing the ruling.