Ibn Sirin narrates from Abdullah bin Shaqiq, from Sayyidah Aisha (may Allah be pleased with her), that she said: The Messenger of Allah (peace and blessings be upon him) used to perform (nafl prayer) both standing and sitting. So when he (peace and blessings be upon him) began the prayer standing, he would also perform the bowing (ruku') standing; and when he began the prayer sitting, he would also perform the bowing sitting. Imam Abu Bakr (may Allah have mercy on him) says that this narration explains all the previous narrations in detail. Accordingly, as per this narration, when a person begins the prayer standing, then sits and recites, it is appropriate and proper for him to recite some while standing and then perform the bowing while still standing. And when he begins his prayer sitting and recites all while sitting, then he should also perform the bowing sitting, following and emulating the action of the Noble Prophet (peace and blessings be upon him).