It is narrated from Sayyiduna Samurah (may Allah be pleased with him) that the Prophet (peace and blessings be upon him), when he delivered a sermon, would raise his hands so high that the whiteness of his (peace and blessings be upon him) armpits could be seen.
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 6903