Musnad Abu Ya'la al-Mawsili 243:75

And from Abu Musa, from the Prophet, may Allah bless him and grant him peace, who said: When the petitioner—and perhaps he said: the petitioner or the person with a need—came to him, he would say: "Intercede, so that you may be rewarded, and Allah will decree upon the tongue of His Prophet what He wills."