It is narrated from Sayyiduna Abdullah bin al-Zubayr (may Allah be pleased with them both) that when the Messenger of Allah (peace and blessings be upon him) prayed, he would place his left hand on his left thigh and the other hand on his right thigh, and raise his finger like this, pointing.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 13838