Musnad Ahmad 25:269

Yazid narrated to me, he said: Muhammad ibn Ishaq informed us, from Dawud ibn al-Husayn, from Ikrimah, from Ibn Abbas, who said: It was said to the Messenger of Allah, peace and blessings of Allah be upon him: "Which of the religions is most beloved to Allah?" He said: "The tolerant Hanifiyyah (monotheism)."