The Prohibition of Seeking a Woman in Marriage who has already been Proposed to by another Muslim unless She Declines his Proposal, or the First Man gives up or Allows him to do so:
The Messenger of Allaah (sallallaahu `alaihi wa sallam) said:
“A man should not ask for the hand of a woman who is already been proposed to by his Muslim brother, unless the first suitor gives her up, or allows him to ask for her hand.”
Reported by Al-Bukhaaree (5142).
