Musnad Abu Ya'la al-Mawsili 117:42

Hadbata narrated to us, Sahl ibn Abi Hazm narrated to us, from Thabit, from Anas ibn Malik, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Whoever Allah has promised a reward for a deed, He will surely fulfill it for him. And whoever He has promised a punishment for a deed, He is in that regard at His own discretion."