It is narrated from Sayyiduna Rifa'ah bin Rafi' (may Allah be pleased with him) that he was sitting in the company of the Messenger of Allah (peace and blessings be upon him) when suddenly a man entered the mosque and prayed. When he had finished his prayer, he came to the Messenger of Allah (peace and blessings be upon him) and greeted him with salam. The Messenger of Allah (peace and blessings be upon him) said: "Wa alayka al-salam (And upon you be peace). Go back and pray, for you have not prayed." Then he mentioned the full hadith of the Prophet (peace and blessings be upon him) teaching him the prayer, and in it is also: "Then he should say the takbir and prostrate, and settle his forehead firmly on the ground, until his joints are at rest and become still."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2645