Source: The Great Interpretation of Dreams (Ibn Sirin)
Breaking a knife in a dream indicates weakness of argument or the death of a child.
Source: Perfuming Humanity in the Expression of Dreams (Al-Nabulsi)
Breaking a knife indicates inability and weakness.
Source: Signs in the Science of Expressions (Ibn Shaheen)
Breaking a knife is interpreted as worry. Whoever sees his knife break will be afflicted with worry.