It is recommended to combine Zhuhr and `Asr prayers at `Arafah, and if one cannot combine these and pray behind an imam, one should combine them while praying alone. Sheikh Sayyed Sabiq, states in his well-known book, Fiqh As-Sunnah: “According to an authentic hadith, the Prophet (peace and blessings be upon him) combined Zhuhr and `Asr prayers while at `Arafah. For this reason, Adhan (call to prayer) was made, followed by Iqamah (the second call to prayer). After that he offered Zhuhr prayer, then another Iqamah was made after which he offered `Asr prayer.
Al-Aswad and `Alqamah said: “For the completion of Hajj, Zhuhr and `Asr prayers must be offered at `Arafah in congregation behind an imam.” Ibn Al-Mundhir said: “There is consensus among the scholars that the imam must combine Zhuhr and `Asr prayers at `Arafah, and so should any one offering prayer with the imam.” In case one cannot combine these and pray behind an imam, one should combine them while praying alone.
It is reported that Ibn `Umar (may Allah be pleased with him) used to reside at Makkah, but when he went to Mina he would shorten his prayers. `Amr Ibn Dinar reported that Jabir Ibn Zaid told him: “Shorten your prayers at `Arafah.” (Reported by Sa`id bin Mansur)”