The World Federation One Stop Fiqh

Ruling 1683

A traveller must not fast if his obligation on a journey is to perform the four unit (rakʿah) prayers as two rakʿahs [i.e. in qaṣr form]. A traveller who performs his prayer in its complete (tamām) form – such as someone whose work is travelling, or someone whose journey is a sinful one – must fast on his journey.