ENOfficial
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."
Source: sunnah.com
Other translations
Arabic
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 .
sunnah.com