What does the source say of rāʾ in مَجْرَاهَا?

Tafkhīm and Tarqīq — Alif, Divine-Name Lām and Rāʾ

• Rāʾ with imālah is light
• Rāʾ is always doubled
• Replace rāʾ with lām
• It demonstrates qalqalah