بِأَيْدِى سَفَرَةٍ
Sentence structure
بِأَيْدِى is a prepositional phrase continuing the description of صُحُفٍ, and سَفَرَةٍ is the genitive complement of أَيْدِى.
Ayah summary
They are in the hands of honoured scribes.
بِأَيْدِى — In (the) hands
Grammar: بِ is a preposition; أَيْدِى is its genitive noun and is also mudāf to سَفَرَةٍ.
سَفَرَةٍۢ — (of) scribes
Grammar: genitive noun functioning as the genitive complement.