What is the ruling on drawing lines in the mosque to facilitate the straightening of the rows of worshippers, and is that considered an innovation (bid'ah)?

It is obligatory to straighten the rows in prayer, due to the Prophet’s (peace and blessings be upon him) saying: "Straighten your rows, for the straightening of rows is part of the establishment of prayer," and his saying: "You must straighten your rows, or Allah will cause discord among your faces." There is no harm in drawing lines on mats or carpets to straighten the rows, and this is not an innovation, especially if the mosque is slightly angled from the Qibla or if people are unable to straighten their rows without them.