Musannaf Ibn Abi Shayba 21:162

Zayd narrated to us, Abd al-Rahman ibn Shurayh narrated to us, Abu Hani narrated to me, from Abu Ali al-Husayn, who said: I heard Abu Sa’id al-Khudri saying: The Messenger of Allah (may Allah bless him and grant him peace) said: "Whoever says: 'I am pleased with Allah as Lord, with Islam as a religion, and with Muhammad as a Messenger,' Paradise becomes mandatory for him."