It is narrated from Umm al-Mu'mineen Sayyidah Aisha (may Allah be pleased with her) that when the Messenger of Allah (peace and blessings be upon him) would pray sitting, he would also recite (Qur'an) while sitting. When thirty or forty verses remained from the recitation, he would stand up and recite while standing, then perform bowing (ruku‘), then prostration (sajdah), and in the second rak‘ah he would do as he did in the first rak‘ah.
Hadith Referenceالسنن المأثورة / ما جاء في الجمع بين الصلاتين في المطر / 28
Hadith Takhrijصحیح بخاری، الصلاة، باب اذا صلى قاعداً ثم صح، رقم: 1119، صحیح مسلم، المساجد ومواضع الصلاة، باب جواز النافلة قائمًا وقاعدًا، رقم: 731