(a) It is narrated from Sayyiduna Anas (may Allah be pleased with him): I saw the Messenger of Allah (peace and blessings be upon him) say the takbir, and he raised his two thumbs up to his ears; then he bowed until every joint settled in its place; then he raised his head from bowing until every joint was straight in its place; then he went down into prostration saying the takbir, until his knees touched the ground before his hands.
(b) An account of placing the knees before the hands, from the practice of Sayyiduna Umar bin al-Khattab and Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with them both).
Al-Ala' bin Isma'il is alone in narrating it, and Allah Almighty knows best.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2632