It is narrated from Sayyiduna Abu Ayyub (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "When one of you goes to relieve himself, do not face the qiblah nor turn your back to it, but face east or west."
Hadith Referenceالمعجم الكبير للطبراني / باب الخاء / 3930