كتاب الإيمان وشرائعه
Read the English translation first, with the original Arabic directly underneath.
It was narrated from Sa'd that :
The Messenger of Allah [ﷺ] distributed (some spoils of war) and gave to some people but withheld from others. I said: "O Messenger of Allah, you gave to so-and-so and so-and-so, and you withheld from so-and-so, who is a beliver." He said: "Do not say 'a believer,' say 'a Muslim.'" Ibn Shihab (one of the narrators) said: The Bedouins say: "We believe".
لاَ تَقُلْ مُؤْمِنٌ وَقُلْ مُسْلِمٌ