(2) The Book on Salat (Prayer)
كتاب الصلاة
(89) Chapter: What Has Been Related About Being Balanced During Prostration
(89) باب مَا جَاءَ فِي الاِعْتِدَالِ فِي السُّجُودِ
Jami` at-Tirmidhi 276 
Anas narrated that :
Allah's Messenger 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."
حَدَّثَنَا مَحْمُودُ بْنُ غَيْلاَنَ، حَدَّثَنَا أَبُو دَاوُدَ، أَخْبَرَنَا شُعْبَةُ، عَنْ قَتَادَةَ، قَالَ سَمِعْتُ أَنَسًا، يَقُولُ إِنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ ‏ "‏ اعْتَدِلُوا فِي السُّجُودِ وَلاَ يَبْسُطَنَّ أَحَدُكُمْ ذِرَاعَيْهِ فِي الصَّلاَةِ بَسْطَ الْكَلْبِ ‏"‏ ‏.‏ قَالَ أَبُو عِيسَى هَذَا حَدِيثٌ حَسَنٌ صَحِيحٌ ‏.‏
Grade :  Sahih (Darussalam)
Reference : Jami` at-Tirmidhi 276
In-book reference : Book 2, Hadith 128
English translation : Vol. 1, Book 2, Hadith 276