Narrated Abu Tameemah: "I was riding behind the Prophet (SAW) when his mount stumbled and so I said, 'May Shaytaan perish!' Upon hearing this the Prophet (SAW) said,
"Do not say, 'May Shaytaan perish', for, when you say this Shaytaan becomes exultant and grows in stature (physique, size) and says, 'It was by my power that I injured him.'
But when you say, "Bismillah" – Shaytan is humiliated, and he reduces to the size of a fly." [Musnad Ahmad (5/59, 365)