From Sahih Bukhari (Hadith 2765):
Narrated `Aisha:The following Verse:-- "If a guardian is well-off, let him claim no remuneration (i.e. wages), but if he is poor, let him have for himself what is just and reasonable." (4.6) was revealed in connection with the guardian of an orphan, and it means that if he is poor he can have for himself (from the orphan's wealth) what is just and reasonable according to the orphan's share of the inheritance
Reference: Bukhari 2765