On the authority of Abu Hurayrah (may Allah be pleased with him), who said that the Messenger of Allah (peace and blessings of Allah be upon him) said:

“When Allah decreed the Creation He pledged Himself by writing in His book which is laid down with Him: ‘My mercy prevails over my wrath.’”

It was related by Muslim (also by al-Bukhari, an-Nasa’i and Ibn Majah).
Hadith Qudsi