Musnad Abu Ya'la al-Mawsili 140:15

Abu Khaythama narrated to us, 'Umar ibn Yunus al-Hanafi narrated to us, 'Ikrimah narrated to us, he said: I heard Yazid al-Raqashi saying: Anas ibn Malik narrated to me, he said: I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "Between the servant and disbelief and polytheism is the abandonment of prayer. So if he abandons the prayer, he has committed disbelief."