Under the Hanafi school, when travel conditions hold, the prescribed fard for Zuhr, ‘Asr and ‘Isha is two rak‘ahs. Al-Shafi‘i regards qasr as a concession, allowing a traveller to perform the full four.
Hanafi evidence includes the report “the traveller’s prayer is two rak‘ahs” and reports from ‘A’ishah about the original number of rak‘ahs. The dispute concerns whether two are definitive or optional relief from four.