The Book of Prayers (Kitab Al-Salat)
Muslim :: Book 4 : Hadith 1783
This hadith has been transmitted thus by 'Umar b. Khattab (with a slight change of words):" I heard Hisham b. Hakim reciting Surah al-Furqan during the lifetime of Allah's Messenger (may peace be upon him)." The rest is the same but with this addition:" I was about to catch hold of him in prayer, but I exercised patience till he pronounced salutation.