It is narrated from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both): The Messenger of Allah (peace and blessings be upon him) was asked which religion is most beloved to Allah. He (peace and blessings be upon him) said: "The gentle and easy Hanifiyyah (pure monotheism)."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 11523