Sunan an Nasai
1158
Sahih+2

It was narrated from Al-Qasim who narrated from 'Abdullah-he is Ibn Abdullah bin 'Umar- that:His father (Ibn 'Umar) said: "One of the sunnahs of the prayer is to hold the right foot upright and point its toes toward the Qiblah, and to sit on the left foot

SahihAbu Ghuddah
SahihAl-Albani