If a person makes the intention in ẓuhr or ʿaṣr prayers that ‘I am performing a four rakʿah prayer’ but does not specify if it is the ẓuhr or ʿaṣr prayer, his prayer is invalid. However, it is sufficient if he specifies the ẓuhr prayer as the first prayer and the ʿaṣr prayer as the second prayer.
With regard to someone for whom it is obligatory, for example, to make up a ẓuhr prayer, if he wants to make up that prayer or perform the ẓuhr prayer within the prescribed time for ẓuhr prayers, he must specify in his intention which prayer he is performing.