It is narrated from Sayyiduna al-Bara' (may Allah be pleased with him) that the Prophet (peace and blessings be upon him), "when he bowed, would keep his back level, and when he prostrated, would turn his fingers towards the qiblah."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2697