It is narrated from Sayyiduna Wa'il bin Hujr (may Allah be pleased with him): I prayed behind the Messenger of Allah (peace and blessings be upon him); then he (peace and blessings be upon him) prostrated, and when going down into prostration the first part of him (peace and blessings be upon him) to touch the ground was his knees.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2631