From Sahih Muslim (Hadith 876):
Mahmud b. al-Rabi', on whose face the Messenger of Allah (ﷺ) squirted water from the well, reported on the authority of 'Ubada b. as- Samit that the Messenger of Allah (ﷺ) said:He who does not recite Umm al-Qur'an is not credited with having observed prayer
Reference: Muslim 876