Musnad Ahmad 409:2

Abdullah ibn Ahmad said: Abu Musa al-Anazi narrated to me, he said: Uthman ibn Umar narrated to us, he said: Sakan ibn al-Mughirah narrated to us, he said: al-Walid ibn Abi Hisham narrated to us, from Farqad Abu Talhah, from Abd al-Rahman ibn Khabbab al-Sulami, who said: I saw the Messenger of Allah, may Allah bless him and grant him peace, deliver a sermon, and he encouraged [the people] to support the Army of Hardship (Jaysh al-Usra), and he mentioned it.