We were sitting with the Prophet ﷺ when a petitioner came and asked him for something. The Messenger of Allah ﷺ said, "Intercede and you will be rewarded, and Allah will decree through the tongue of His Prophet whatever He wills."
حدثنا وكيع حدثنا
كنا جلوسا عند النبي ﷺ وإنه سأله سائل فقال رسول الله ﷺ اشفعوا تؤجروا وليقض الله عز وجل على لسان نبيه ما أحب
Its chain of transmission is sahih. It is found in Bukhari 2/140 in the Book of Friday Prayer, chapter on encouraging charity, Abu Dawud 5/347, number 5132, in the Book of Manners, chapter on intercession, and Nasai 5/78 in the Book of zakat, chapter on interceding for charity.