Du'a after studying

اللَّهُمَّ إِنِّي أَسْتَوْدِعُكَ مَا قَرَأْتُ وَمَا حَفِظْتُ فَرُدَّهُ عَلَيَّ عِنْدَ حَاجَتِي إِلَيْهِ إِنَّكَ عَلَى مَا تَشَاءُ قَدِيرٌ وَأَنْتَ حَسْبِي وَنِعْمَ الْوَكِيلُ

Allahumma inni astawdi'uka ma qara'tu wa ma hafizhtu faruddahu 'alayya 'inda hajati ilayh, innaka 'ala ma tasha'u qadir wa anta hasbi wa ni'mal-wakil

O Allah, I entrust You with what I have read and what I have memorised. Bring it back to me when I am in need of it. You are capable of whatever You will, and You are sufficient for me, and the best disposer of affairs.

A general supplication, not a narration from the Prophet (peace be upon him).