Musnad Ahmad 652:45

Ibn Numayr narrated to us, al-A‘mash narrated to us, from Dharr, from Yusi‘, from al-Nu‘man ibn Bashir, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Supplication is worship." Then he recited: "Call upon Me; I will respond to you" [Ghafir: 60].