During the prostrations one should keep one's arms away from one's sides and the abdomen should be kept away from the thighs.

Sahih al-Bukhari · Book 10 · Hadith 807

حَدَّثَنَا يَحْيَى بْنُ بُكَيْرٍ، قَالَ حَدَّثَنِي بَكْرُ بْنُ مُضَرَ، عَنْ جَعْفَرٍ، عَنِ ابْنِ هُرْمُزَ، عَنْ عَبْدِ اللَّهِ بْنِ مَالِكٍ ابْنِ بُحَيْنَةَ، أَنَّ النَّبِيَّ صلى الله عليه وسلم كَانَ إِذَا صَلَّى فَرَّجَ بَيْنَ يَدَيْهِ حَتَّى يَبْدُوَ بَيَاضُ إِبْطَيْهِ‏.‏ وَقَالَ اللَّيْثُ حَدَّثَنِي جَعْفَرُ بْنُ رَبِيعَةَ نَحْوَهُ‏.‏

Narrated `Abdullah bin Malik bin Buhaina: Whenever the Prophet (ﷺ) used to offer prayer he used to keep arms away (from the body) so that the whiteness of his armpits was visible.