Muwatta Malik 368

وَحَدَّثَنِي عَنْ مَالِكٍ، أَنَّهُ بَلَغَهُ أَنَّ سَعْدَ بْنَ أَبِي وَقَّاصٍ، كَانَ يَمُرُّ بَيْنَ يَدَىْ بَعْضِ الصُّفُوفِ وَالصَّلاَةُ قَائِمَةٌ ‏.‏ قَالَ مَالِكٌ وَأَنَا أَرَى ذَلِكَ وَاسِعًا إِذَا أُقِيمَتِ الصَّلاَةُ وَبَعْدَ أَنْ يُحْرِمَ الإِمَامُ وَلَمْ يَجِدِ الْمَرْءُ مَدْخَلاً إِلَى الْمَسْجِدِ إِلاَّ بَيْنَ الصُّفُوفِ ‏.‏

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