Musnad Abu Ya'la al-Mawsili 152:12

Muhammad ibn Isma'il ibn Abi Samina al-Basri narrated to us, Muhammad ibn Khalid al-Hanafi narrated to us, Musa ibn Ya'qub al-Zam'i narrated to us, from 'Abdullah ibn Kaysan, from 'Abdullah ibn Shaddad, from 'Abdullah ibn Mas'ud, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Indeed, the closest of you to me on the Day of Resurrection is the one among you who sends the most blessings upon me."