Musnad Ahmad 652:86

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