Musannaf Ibn Abi Shayba 33:1037

Zayd ibn al-Hubab narrated to us, from Kathir ibn Zayd al-Madini, who said: Al-Muttalib ibn Abdullah ibn Hantab narrated to me, from Amir ibn Sa’d ibn Abi Waqqas, who said: I met Abu Ayyub al-Ansari, and he said to me: Shall I not command you with what the Messenger of Allah (may Allah bless him and grant him peace) commanded me? That I should frequently say: "There is no power and no strength except by Allah" (la hawla wa la quwwata illa billah), for it is a treasure from the treasures of Paradise.