Sayyiduna Samurah (may Allah be pleased with him) says: The Messenger of Allah (peace and blessings be upon him) used to order us that when we are in the prayer and raise our heads from prostration, we should sit on the ground with composure, and not crouch up on the tips of our toes.
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 6990