It is from the sunnah of the prayer that you lay your left foot flat and keep your right foot upright.
أخبرنا قتيبة بن سعيد قال حدثنا الليث عن يحيى عن القاسم بن محمد
إن من سنة الصلاة أن تضجع رجلك اليسرى وتنصب اليمنى
Bukhari recorded it in the Book of adhan, chapter on the sunnah of sitting during tashahhud, no. 827, from the hadith of Abdullah bin Abdullah bin Umar. Abu Dawud recorded it in the Book of Prayer, chapter on how to sit during tashahhud, no. 960, 959, from the hadith of Yahya bin Saeed Ansari, with the same wording. It is also found in Kubra, no. 743.