Hazrat Moulana Muhammad Ilyaas (rahmatullahi ‘alaih) once mentioned:

The position of a fardh action is higher than that of a nafl action. In fact, the very purpose of a nafl action is to perfect the fardh action or to compensate for any weakness or shortcoming which entered the fardh during its performance. In essence, fardh actions are the primary acts of worship and nafl actions are secondary acts of worship. However, despite the fact that priority should be given to the fardh actions, there are people who neglect the performance of fardh actions and instead show more importance to remaining engaged in nafl actions. (Malfoozaat Hazrat Moulana Muhammad Ilyaas (rahmatullahi ‘alaih) pg. 13)

Source: Ihyaauddeen.co.za