The phrase is from the Holy Bible, in the Gospel According To Mark, 10:9 (the words of Jesus) ~ What therefore God hath joined together, let not man put asunder.
In my ceremonies I tend to place it in the pronouncement worded as follows … “Therefore what God hath joined together, let no man put asunder.”
It can also be in the wedding prayer/blessing or after the exchange of rings.
Some other versions are:
“Therefore what God has joined together, let man not separate."
“What therefore God has joined together, let not man separate.”
"What therefore God has joined together, let no man separate."
“What therefore God has joined together, let not man put asunder.”
“What therefore God hath joined together, let not man put asunder.”
To read more about wedding ceremony elements and for complete wedding ceremonies that include the familiar “let no man put asunder” line, see the Ceremonies and Elements page.