مَرَرْتُ بِعُمَرَ
Translation: I passed by ʿUmar.
مَرَرْتُ: past-tense verb with attached first-person subject.
بِ: preposition.
عُمَرَ: genitive noun governed by bi-, taking fatḥa instead of kasra because it is a diptote.
The prepositional phrase relates to «مَرَرْتُ». Grammatical genitive case must not be confused with a visible kasra.