Sayyiduna Abu Hurayrah (may Allah be pleased with him) said that the Prophet (peace and blessings be upon him) said: "When one of you prays, let him not place his sandals on his right or on his left; if he places them on his left, they will be on someone else's right. Yes, he may place them on his left when there is no other person on his left; otherwise, let him place them between his two feet."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4259