Muwatta Malik 368

Yahya related to me from Malik that he had heard that Said ibn Abi Waqqas used to pass in front of some of the rows while the prayer was in progress. Malik said, "I consider that it is permissible to do that if the iqama for the prayer has been said and the imam has said the initial takbir and a man cannot find any way into the mosque except by going between the rows