It is narrated from Hazrat Aisha (may Allah be pleased with her) that the Prophet (peace and blessings be upon him) used to pray standing and also used to pray sitting. When he prayed standing, he would also perform the bowing (ruku) standing, and when he prayed sitting, he would also perform the bowing sitting.
Hadith Referenceمسند احمد / مسند النساء / 26274
Hadith Gradingحکم دارالسلام:حديث صحيح، وهذا إسناد ضعيف لضعف ليث بن أبى سليم