Musnad Abu Ya'la al-Mawsili 140:41

Abu al-Rabi' narrated to us, Hammad narrated to us, al-Mu'alla ibn Ziyad narrated to us, from Yazid al-Raqashi, from Anas ibn Malik, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "That I sit with a people who remember Allah from the afternoon prayer (Asr) until the sun sets is more beloved to me than that I free eight of the children of Ishmael."