Sahih BukhariVolume 5, Book 59, Number 615

Narrated 'Abdullah bin Amr:

When Allah's Apostle besieged Taif and could not conquer its people, he said, "We will return (to Medina) If Allah wills." That distressed the Companions (of the Prophet and they said, "Shall we go away without conquering it (i.e. the Fort of Taif)?" Once the Prophet said, "Let us return." Then the Prophet said (to them), "Fight tomorrow." They fought and (many of them) got wounded, whereupon the Prophet said, "We will return (to Medina) tomorrow if Allah wills." That delighted them, whereupon the Prophet smiled. The sub-narrator, Sufyan said once, "(The Prophet) smiled."