Which is better for a person who has not prayed 'Asr and finds a congregation praying a delayed Dhuhr and 'Asr (Jam' Ta'khir): should he join them for Dhuhr with the intention of 'Asr, or should he wait for them to finish Dhuhr and then join them for 'Asr?

"The congregant is not required to have the same intention as the Imam. Thus, it is permissible for someone performing a supererogatory prayer to pray behind someone performing an obligatory prayer, and for someone performing an obligatory prayer to pray behind someone performing a different obligatory prayer. This view has been adopted by groups of scholars, including Ata, Tawus, Al-Shafi'i, and Ahmad ibn Hanbal. Nevertheless, if there is no hardship involved, it is better for the worshiper to wait for a congregation that aligns with their obligatory prayer, in order to avoid disagreement and achieve the perfection of following the Imam."