Sunan Abi Dawud Book 2, Hadith 227
The Prophet (ﷺ) said: When the Imam completes the prayer and sits (for reciting tashahhud), and then becomes defiled (i.e. his ablution becomes void) before he speaks (to someone), his prayer becomes complete. And those who prayed behind him also complete the prayer.
The Prophet (ﷺ) said: The Imam should not pray at the place where he led the prayer until he removes (from there).
Abu Dawud said: 'Ata' al-Khurasani did not see Mughirah b. Shu'bah (This tradition is, therefore munqati', i.e. a link is missing in the chain).
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.