Musnad Ahmad 32:220

Yahya narrated to us, from Sufyan, who said: Al-Qasim ibn Shurayh narrated to me, from Tha'labah, who said: I heard Anas saying: I heard the Prophet, may Allah bless him and grant him peace, saying: "I am amazed at the believer; for Allah does not decree a decree, except that it is good for him."