Switching Between Sitting and Standing:

• The Messenger of Allah (saw) used to make nightly voluntary prayer (Tahajjut) generally in stading position, but when he was old, he would sometimes pray sitting.
• When praying sitting, sometimes he would stand up to complete his recitation and go into Ruku, an at other times, he would go into Ruku in sitting position.
• Sometime the prophet (saw) used to switch between sitting and standing while praying.