Musnad Abu Ya'la al-Mawsili 115:98

Nasr ibn Ali al-Jahdhami narrated to us, Nuh ibn Qays narrated to us, from his brother Khalid ibn Qays, from Qatadah, from Anas, who said: A man said, "O Messenger of Allah, how many prayers has Allah obligated upon His servants?" He said, "Five prayers." He said, "Is there anything before them or after them?" He said, "Allah has obligated five prayers upon His servants." The man swore by Allah that he would not add to them nor decrease from them. The Messenger of Allah, peace and blessings of Allah be upon him, said, "If he is truthful, he will enter Paradise."