«إِلَى أَنْ وَقَفْتُ عَلَى بَابِ دَارٍ وَقُلْتُ عَلَى بِدَارٍ»
Until I stood at a house door and immediately spoke out.
Bāb dār locates the speaker at a house door; ʿalā bidār expresses promptness. The shared ending of dār / bidār creates a cadence that carries the listener into the following verse.
One phrase gives place, the other manner and speed. Their phonetic correspondence is therefore integrated with a narrated sequence, rather than being mere decoration.
Analysis: door indicates location; immediately indicates manner and speed.