It is narrated from Umm al-Mu'minin Sayyida Aisha (may Allah be pleased with her) that she never saw the Messenger of Allah (peace and blessings be upon him) performing prayer while sitting, until when he became old, he would pray sitting and would also recite while sitting. But when he intended to bow (for ruku'), he would stand up and recite thirty or forty more verses, then he would perform ruku'.
Hadith Referenceالسنن المأثورة / ما جاء في الجمع بين الصلاتين في المطر / 27
Hadith Takhrijصحیح بخاری، الصلاة، باب اذا صلى قاعداً ثم صح، رقم: 1118، صحیح مسلم، المساجد ومواضع الصلاة، باب جواز النافلة قائما و قاعدًا، رقم: 731