Hazrat Moulana Ashraf Ali Thanwi (rahmatullahi ‘alaih) once mentioned:

Zikr is a means of acquiring great blessings. However, one will derive the full benefit from the blessings provided one refrains from evil actions and sins. This could be understood through the following illustration. Consider the example of a person who does not perform his faraaidh and performs nafl salaah. Though he will receive reward through performing the nafl salaah, however the sin of neglecting the faraaidh will outweigh the reward received through the nafl, to such an extent that the benefit of the nafl will not be seen. Furthermore, he will not find any stability and firmness in all other righteous actions. (Malfoozaat Hakeemul Ummat 2/164)

Source: Ihyaauddeen.co.za