Sunan Abi Dawud Book 2, Hadith 229
The Prophet (ﷺ) said: Do not try to outstrip me in bowing and prostrating because however earlier I bow you will join me when I raise (my head from bowing); I have become bulky.
The Prophet (ﷺ) said: The key of prayer is purification; takbir (saying "Allah is most great") makes (all acts which break prayer) unlawful and taslim (uttering the salutation) makes (all such acts) lawful.
Al-Bara who is not a liar said; when they (the companions) raised their heads from bowing along with the Messenger of Allah (ﷺ), they would stand up, and when they saw him that he went down in prostration, they would prostrate (following the prophet).