Abu Dawud Book 2, Hadith Number 617.
Chapter : Not known.
Narated By Abdullah ibn Amr ibn al-‘As : The Prophet (PBUH) 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.