Musnad Ahmad 788:183

Yahya ibn Sa'id narrated to us, from Sufyan, Abu Burdah ibn 'Abdullah ibn Abi Burdah narrated to me, from his grandfather, from Abu Musa al-Ash'ari, who said: When a petitioner or someone in need came to the Messenger of Allah (peace and blessings of Allah be upon him), he would say: "Intercede, and you will be rewarded, and let Allah, the Almighty and Majestic, decree upon the tongue of His Messenger whatever He wills." And he said: "The believer to the believer is like a structure, parts of which strengthen other parts." And he said: "The trustworthy treasurer who delivers what he is commanded to deliver, with a willing soul, is one of the two givers of charity."