The Virtue of Al-Fatihah
Imam Ahmad recorded in his Musnad, that a person who was riding behind the Prophet said, "The Prophet's animal tripped, so I said, `Cursed Shaytan.' The Prophet said,
«لَا تَقُلْ: تَعِسَ الشَّيْطَانُ، فَإِنَّكَ إِذَا قُلْتَ: تَعِسَ الشَّيْطَانُ، تَعَاظَمَ وَقَالَ: بِقُوَّتِي صَرَعْتُهُ، وَإِذَا قُلْتَ: بِاسْمِ اللهِ تَصَاغَرَ حَتى يَصِيرَ مِثْلَ الذُبَابِ»
(Do not say, 'Cursed Shaytan,' for if you say these words, Satan becomes arrogant and says, 'With my strength I made him fall.' When you say, 'Bismillah,' Satan will become as small as a fly.)
Further, An-Nasa'i recorded in his book Al-Yawm wal-Laylah, and also Ibn Marduwyah in his Tafsir that Usamah bin `Umayr said, "I was riding behind the Prophet...'' and he mentioned the rest of the above Hadith. The Prophet said in this narration,
«لَا تَقُلْ هكَذَا فَإِنَّهُ يَتَعَاظَمُ حَتَّى يَكُونَ كَالْبَيْتِ، وَلكِنْ قُلْ: بِسْمِ اللهِ، فَإنَّهُ يَصْغَرُ حَتَّى يَكُونَ كَالذُبَابَةِ»
(Do not say these words, because then Satan becomes larger; as large as a house. Rather, say, 'Bismillah,' because Satan then becomes as small as a fly.)
This is the blessing of reciting Bismillah.