The sunnah of prayer is that you set your right foot upright and fold your left foot under you.
حدثنا عبد الله بن مسلمة عن مالك عن عبد الرحمن بن القاسم عن عبد الله بن عبد الله
سنة الصلاة أن تنصب رجلك اليمنى وتثني رجلك اليسرى
Bukhari narrated it in "Adhan," chapter on the sunnah of sitting during the tashahhud, hadith no. 827, from Abdullah bin Maslama Qanabi, and it is found in Muwatta (Yahya): 1/89, 90.