Are the invocations (dhikr) and supplications (du'a) outside of prayer obligatory?

The remembrances (adhkar) and supplications (du'as) recited outside of prayer—such as the morning and evening adhkar, those recited after prayer, at bedtime, upon waking, when entering and exiting, and when riding and dismounting—are recommended (mustahabb) but not obligatory (wajib). Supplications may be obligatory in some cases, such as the supplication within Surat al-Fatiha.