It was the practice of the Prophet (peace and blessings be upon him):
"When he (peace and blessings be upon him) bowed, he would spread his fingers apart, and when he prostrated, he would keep his fingers together."
Hadith Referenceصحیح الجامع الصغیر / حرف الكاف / 4733
Hadith Gradingالألبانی:صحيح
Hadith Takhrij[ك هق] عن وائل بن حجر. صفة الصلاة ١١٠، ١٢٣، صحيح أبي داود ٨٠٩: الطيالسي.