وَلَا بُدَّ مِنْ عَائِدٍ فِيهَا يَعُودُ إِلَى الْمَوْصُولِ.
Translation: The ṣilah must contain a returning element referring back to the relative noun.
The ʿāʾid links the relative noun to its clause. It may be overt, understood as a subject pronoun, or omitted under specific rules.
In «الَّذِي أَبُوهُ قَائِمٌ», the «هُ» of «أَبُوهُ» is the returning pronoun.