Ibrahim (may Allah have mercy on him) said: When a man makes a bequest for someone that he should be given a share like that of his son, then his share will not be complete until there is a reduction from the son's share.
Hadith Referenceسنن دارمي / من كتاب الوصايا / 3283
Hadith Takhrijاس اثر کی سند صحیح ہے۔ دیکھئے: [ابن أبى شيبه 10844]
Brief Explanation
(Commentary on Hadith 3282)
That is, when a person is given a share like that of a son, then certainly the son's share will be reduced. This is why people used to dislike such a bequest.