Whenever the Prophet (may Allah's peace and blessings be upon him) bowed, he would spread out his fingers, and whenever he prostrated, he would join them together
Authentic hadithعن وائل بن حجر رضي الله عنه : أن النبي صلى الله عليه وسلم كان إذا ركع فَرَّجَ أصابِعَه وإذا سَجد ضَمَّ أصَابِعه.
Wā’il ibn Hujr (may Allah be pleased with him) reported: Whenever the Prophet (may Allah's peace and blessings be upon him) bowed, he would spread out his fingers, and whenever he prostrated, he would join them together.
Explanation
The Hadīth indicates that when the Prophet (may Allah's peace and blessings be upon him) bowed, he would hold his knees with his palms and spread out his fingers, as this posture helps a person bow properly and straighten his back at the same level with his head. And in prostration, the Prophet (may Allah's peace and blessings be upon him) would place his palms on the ground and join his fingers together, leaving no space between them, so that he could face the Qiblah perfectly. This also makes prostration less burdensome.
Source · Narrated by Ibn Hebban
From the Hadith Encyclopedia (hadeethenc.com).