Ibn Ishaq narrated a hadith about the Messenger of Allah (peace and blessings be upon him) keeping his heels and his toes upright between the two prostrations. Abdullah bin Abi Najih narrates from Mujahid bin Jabr that he heard Abdullah bin Abbas (may Allah be pleased with them both) mention this, and he said to him: "O Abu al-Abbas! By Allah, we used to count this as uncouth on the part of whoever did it." He said: "It is Sunnah."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2734