بُلُوغُ الْمَرَامِ
Bulugh al-Maram 300
The Book of Prayer
وَعَنْ وَائِلِ بْنِ حُجْرٍ - رضى الله عنه - { أَنَّ اَلنَّبِيَّ - صلى الله عليه وسلم -كَانَ إِذَا رَكَعَ فَرَّجَ بَيْنَ أَصَابِعِهِ , وَإِذَا سَجَدَ ضَمَّ أَصَابِعَهُ } رَوَاهُ اَلْحَاكِمُ . [1]
Narrated Wa'il bin Hujr (RA): Whenever the Prophet (ﷺ) bowed, he would spread out his fingers and when he prostrated he would bring his fingers together. [Reported by al-Hakim).
Sunnah.com reference: Book 2, Hadith 186