مُسْنَدُ أَحْمَدَ
Musnad Ahmad 2105
Musnad of 'Abdullah bin al-'Abbas bin 'Abdul-Muttalib from the Prophet
It was narrated that Ibn 'Abbas (رضي الله عنه) said: It was said to the Messenger of Allah (ﷺ): Which religion is dearest to Allah? He said: "Easy monotheism."
Musnad Ahmad 2107Saheeh because of corroborating evidence]