ENOfficial
Abu Hurairah reported the Prophet(ﷺ) as saying: when one of you prostrates himself, he should not stretch out his forearms( on the ground) like a dog and he should join both of his thighs.
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا عَبْدُ الْمَلِكِ بْنُ شُعَيْبِ بْنِ اللَّيْثِ، حَدَّثَنَا ابْنُ وَهْبٍ، حَدَّثَنَا اللَّيْثُ، عَنْ دَرَّاجٍ، عَنِ ابْنِ حُجَيْرَةَ، عَنْ أَبِي هُرَيْرَةَ، أَنَّ النَّبِيَّ صلى الله عليه وسلم قَالَ " إِذَا سَجَدَ أَحَدُكُمْ فَلاَ يَفْتَرِشْ يَدَيْهِ افْتِرَاشَ الْكَلْبِ وَلْيَضُمَّ فَخِذَيْهِ " .
sunnah.com