Narrated by Sayyiduna Uqbah bin Amir (may Allah be pleased with him) that the Noble Prophet (peace and blessings be upon him) said: The conditions which deserve to be fulfilled the most are those by which you make private parts lawful (i.e., marriage contracts).
Narrated by Sayyiduna Abu Huraira (may Allah be pleased with him) that the Noble Prophet (peace and blessings be upon him) said: No woman should stipulate the divorce of her sister.
It is narrated from Sayyiduna Abdullah bin Amr (may Allah be pleased with them both) that the Noble Prophet (peace and blessings be upon him) said: It is not lawful that a woman be divorced in order to marry another woman.
Narrated by Sayyidah Aisha (may Allah be pleased with her) that the Messenger of Allah (peace and blessings be upon him) said: Every condition that is not in the Book of Allah is rejected, even if people make a hundred such conditions.