Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 773
Mahmud b. al-Rabi', on whose face the Messenger of Allah (may peace be upon him) squirted water from the well, reported on the authority of 'Ubada b. as- Samit that the Messenger of Allah (may peace be upon him) said: He who does not recite Umm al-Qur'an is not credited with having observed prayer.