Sayyidah Aisha Siddiqa (may Allah be pleased with her) narrates: The Prophet Muhammad (peace be upon him) used to perform long prayers at night while standing, and he also used to perform long prayers while sitting. When he performed the prayer standing, he would go into bowing (ruku) from the standing position, and when he performed the prayer sitting, he would go into bowing from the sitting position.
Hadith Referenceصحیح ابن حبان / كتاب الصلاة / 2631
Hadith Gradingفضيلة الشيخ الإمام محمد ناصر الدين الألبانيصحيح - «صفة الصلاة»، «مختصر الشمائل المحمدية» (236)، «صحيح أبي داود» (880 و 1137): م.فضيلة الشيخ العلّامة شُعيب الأرناؤوطإسناده صحيح على شرط مسلم