The Qur’an states, as part of the ruling on accidental killing:
«فَتَحْرِيرُ رَقَبَةٍ مُؤْمِنَةٍ»
Then freeing a believing enslaved person is required. (An-Nisa 4:92)
The noun raqabah names the kind of person to be emancipated. The following adjective mu’minah adds the requirement of being a believer.
This is not merely a longer expression; the additional word narrows what can fulfil this particular expiation.