Sunan an-Nasa'i, Book 12, Hadith 129 (In-Book) ⓘ
It was narrated from 'Abdullah bin 'Abdullah bin 'Umar that his father said: "One of the sunnahs of the prayer is to spread your left foot beneath you, and hold your right foot upright."
أَخْبَرَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، قَالَ حَدَّثَنَا اللَّيْثُ، عَنْ يَحْيَى، عَنِ الْقَاسِمِ بْنِ مُحَمَّدٍ، عَنْ عَبْدِ اللَّهِ بْنِ عَبْدِ اللَّهِ بْنِ عُمَرَ، عَنْ أَبِيهِ، أَنَّهُ قَالَ إِنَّ مِنْ سُنَّةِ الصَّلاَةِ أَنْ تُضْجِعَ رِجْلَكَ الْيُسْرَى وَتَنْصِبَ الْيُمْنَى .
also referenced under: Hadith 1157 (Reference) & Volume 2, Book 12, Hadith 1158 (DarusSalam)