Prayer (Kitab Al-Salat)

Dawud :: Book 2 : Hadith 618

Narrated Ali ibn AbuTalib:

The Prophet (peace_be_upon_him) 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.