Musnad Ahmad 652:90

'Abd al-Rahman narrated to us, from Sufyan, from Mansur and al-A'mash, from Dharr, from Yusi' al-Hadrami, from al-Nu'man ibn Bashir, who said: I heard the Messenger of Allah, peace and blessings of Allah be upon him, delivering a sermon and saying: "Verily, supplication is worship." Then he recited: "And your Lord says, 'Call upon Me; I will respond to you'" [Ghafir: 60].