From Sahih Bukhari (Hadith 3912):
Narrated Ibn `Umar:`Umar bin Al-Khattab fixed a grant of 4000 (Dirhams) for every Early Emigrant (i.e. Muhajir) and fixed a grant of 3500 (Dirhams) only for Ibn `Umar. Somebody said to `Umar, "Ibn `Umar is also one of the Early Emigrants; why do you give him less than four-thousand?" `Umar replied, "His parents took him with them when they migrated, so he was not like the one who had migrated by himself
Reference: Bukhari 3912