Sunan An-Nasa'i

It was narrated from 'Abdullah bin 'Utbah bin Mas'ud that a man came to the Prophet and said:""I have given a gift to my son, so bear witness."" He said: ""Do you have any other children?"" He said: ""Yes."" He said: ""Have you given them something like that which you have given him?"" He said: ""No."" He said: ""Shall I bear witness to unfairness?