Sayyiduna Wa'il bin Hujr (may Allah be pleased with him) narrates: When the Prophet (peace and blessings be upon him) went into bowing, he would keep his fingers spread apart, and when he went into prostration, he would keep his fingers together.
Imam al-Tabarani recorded it in al-Mu'jam al-Kabir, and its chain is hasan (good).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصلاة / 2807