Should a worshiper complete the Asr prayer if the Maghrib call to prayer is made while he is in the last raka'ah?

If a worshipper catches one unit (rak'ah) of the Asr prayer before sunset, then they have caught it, and their prayer is considered performed on time (ada'). They must complete their prayer, and it is not permissible to interrupt a fard (obligatory) prayer after commencing it except for an excuse, due to Allah's saying: "O you who have believed, obey Allah and obey the Messenger and do not invalidate your deeds."