It is narrated from Sayyiduna Wa'il bin Hujr (may Allah be pleased with him) that he said: "I saw that when the Messenger of Allah (peace and blessings be upon him) began the prayer, he would raise his hands until the tips of his (peace and blessings be upon him) thumbs were level with his (peace and blessings be upon him) earlobes."
Hadith Referenceالمعجم الكبير للطبراني / باب الواو / 18292