Book 11, Number 3971:

Narrated Jabir ibn Abdullah:

The wife of Bashir said (to her husband): Give to my son your slave as a gift, and make for me Allah"s Messenger (peace_be_upon_him) a witness.

He came to Allah"s Messenger (peace_be_upon_him) and said: The daughter of so and so (his wife Amrah bint Rawahah) asked me to give my slave as a gift to her son, and call for me Allah"s Messenger (peace_be_upon_him) as a witness.

Thereupon he (the Prophet) said: Has he (Nu"man) brothers? He (Bashir) said: Yes. He (further) said: Have you given to all others as you have given to him? He said: No. He said: Then it is not fair; and verily I cannot bear witness but only to what is just.