The Mother of the Believers, Sayyidah Aishah (may Allah be pleased with her), narrates that the Messenger of Allah (peace and blessings be upon him) used to pray both standing and sitting. When he prayed standing, he also bowed (ruku') standing, and when he prayed sitting, he also bowed sitting.
Hadith Referenceالمعجم الأوسط / باب الألف / 2471
Hadith Takhrij«أخرجه أحمد فى «مسنده» برقم: 26915، وأبو يعلى فى «مسنده» برقم: 4795، والطحاوي فى «شرح معاني الآثار» برقم: 1989»