إِنَّ أَسْرَعَ الدُّعَاءِ إِجَابَةً دَعْوَةُ غَائِبٍ لِغَائِبٍ
Translation: Among supplications, the one most swiftly answered is the prayer of an absent person for another absent person.
Source: الترمذي
Praying for someone who cannot hear or reward the prayer can express sincere goodwill. غَائِبٍ لِغَائِبٍ describes a person praying for another person who is absent. إِجَابَةً points to the answering of supplication. This expresses merit, not a guaranteed timetable for the result of any particular prayer.
أَسْرَعَ is a comparative/superlative form meaning “swifter.” الدُّعَاءِ completes the iḍāfah, while دَعْوَةُ is the predicate and غَائِبٍ its genitive complement. لِغَائِبٍ specifies the person for whom the prayer is made.
Praying privately for friends, parents, the sick, or people in hardship is a direct application.