Musannaf Ibn Abi Shayba 33:1035

Sufyan ibn 'Uyaynah narrated to us, from Ibn al-Sa'ib ibn Yazid, from 'Amr ibn Maymun, that he heard Abu Dharr saying: "I was walking behind the Prophet, may God bless him and grant him peace, and he said: 'Shall I not guide you to a treasure from the treasures of Paradise?' I said: 'Yes.' He said: 'There is no power and no strength except by God (la hawla wa la quwwata illa billah).'"