the Prophet said: "When one of you prostrates, then let him be balanced, and let him not lay his forearms down like the lying of the dog."  

أَنَّ النَّبِيَّ ﷺ قَالَ إِذَا سَجَدَ أَحَدُكُمْ فَلْيَعْتَدِلْ وَلاَ يَفْتَرِشْ ذِرَاعَيْهِ افْتِرَاشَ الْكَلْبِ قَالَ وَفِي الْبَابِ عَنْ عَبْدِ الرَّحْمَنِ بْنِ شِبْلٍ وَأَنَسٍ وَالْبَرَاءِ وَأَبِي حُمَيْدٍ وَعَائِشَةَ  

Messenger of Allah said: "Be balanced in the prostration, and let one of you not spread his forearms (on the ground) in the Salat like the spreading of the dog."  

سَمِعْتُ أَنَسًا يَقُولُ إِنَّ رَسُولَ اللَّهِ ﷺ قَالَ اعْتَدِلُوا فِي السُّجُودِ وَلاَ يَبْسُطَنَّ أَحَدُكُمْ ذِرَاعَيْهِ فِي الصَّلاَةِ بَسْطَ الْكَلْبِ