The Book of Pilgrimage (Kitab Al-Hajj)

Muslim :: Book 7 : Hadith 2954

Ibn 'Umar rep rte that Allah's Messenger (may peace be upon him) combined the sunset and 'Isha ' prayers at Muzdalifa. He observed three rak'ahs of the sunset prayer and two rak'ahs of the 'Isha' prayer with one Iqama.

Source materials are from the University of Southern California MSA site
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