The Noble Prophet (s.a.w) had promised one of his companions, Abu Haitham Ibn Tayyahan, that he (s.a.w) would present him with a slave. Incidentally it so happened that three captives were brought before him – two of whom he (s.a.w) gifted away while one remained with him.
Meanwhile Hadhrat Zahra (s.a) approached him (s.a.w) and requested: “O’ Prophet of Allah! Can you grant me a slave or an assistant? Do you not see the effects of the hand-mill upon my hands?” At that moment the Noble Prophet (s.a.w) suddenly recollected his promise to Abu Haitham and said to himself: “Since I had previously promised Abu Haitham, how can I grant my daughter precedence over him, even though my daughter turns the hand-mill with her weak and delicate hands?”( Shanidaniha-e-Tarikh, pg. 290; Mahajjah al-Baidha, vol. 5, pg. 338 )