Yahya related to me from Malik from Nafi that Umar ibn al-Khattabsaid, "Any direction that is between east and west can be taken as aqibla if the person praying is face-on to the House." (RU)
USC-MSA web (English) reference: Book 14, Hadith 8 (RU)
Arabic reference (RU): Book 14, Hadith 465 (RU)