I heard Ibn al-Zubair say: Setting the feet right and placing one hand on the other is a sunnah.  

سَمِعْتُ ابْنَ الزُّبَيْرِ يَقُولُ صَفُّ الْقَدَمَيْنِ وَوَضْعُ الْيَدِ عَلَى الْيَدِ مِنَ السُّنَّةِ  

AbuUthman an-Nahdi said: When Ibn Mas'ud prayed he placed his left hand on the right. The Prophet ﷺ saw him and placed his right hand on his left one.  

أَنَّهُ كَانَ يُصَلِّي فَوَضَعَ يَدَهُ الْيُسْرَى عَلَى الْيُمْنَى فَرَآهُ النَّبِيُّ ﷺ فَوَضَعَ يَدَهُ الْيُمْنَى عَلَى الْيُسْرَى  

AbuJuhayfah said: Ali said that it is a sunnah to place one hand on the other in prayer below the navel.  

السُّنَّةُ وَضْعُ الْكَفِّ عَلَى الْكَفِّ فِي الصَّلاَةِ تَحْتَ السُّرَّةِ  

I saw Ali (Allah be pleased with him) catching hold of his left hand) by his right hand on the wrist above the navel. Abu Dawud said: Sa'id b. Jubair narrated the words: "above the navel". Abu Mijlaz reported the words: "below the navel". This has also been narrated by Abu Hurairah. But that is not strong.  

رَأَيْتُ عَلِيًّا ؓ يُمْسِكُ شِمَالَهُ بِيَمِينِهِ عَلَى الرُّسْغِ فَوْقَ السُّرَّةِ  

(The established way of folding hands is) to hold the hands by the hands in prayer below the navel. Abu Dawud said: I heard Ahmad b. Hanbal say: The narrator 'Abd al-Rahman b. Ishaq al-Kufi is weak (i.e. not reliable).  

أَبُو هُرَيْرَةَ أَخْذُ الأَكُفِّ عَلَى الأَكُفِّ فِي الصَّلاَةِ تَحْتَ السُّرَّةِ  

The Messenger of Allah ﷺ used to place his right hand on his left hand, then he folded them strictly on his chest in prayer.  

كَانَ رَسُولُ اللَّهِ ﷺ يَضَعُ يَدَهُ الْيُمْنَى عَلَى يَدِهِ الْيُسْرَى ثُمَّ يَشُدُّ بَيْنَهُمَا عَلَى صَدْرِهِ وَهُوَ فِي الصَّلاَةِ