أَوْ بِمَعْنَى «فِي»، نَحْوُ: «صَلَاةُ اللَّيْلِ».
Translation: Or it may convey the sense of fī, as in “night prayer.”
صَلَاةُ is mudāf and اللَّيْلِ is a genitive mudāf ilayh.
Its sense is like صَلَاةٌ فِي اللَّيْلِ, prayer during the night. The implied fī gives a time relationship.
Added example: سَفَرُ الصَّبَاحِ, a journey made in the morning.