It is narrated from Muhammad bin Muslim that he said: One day I prayed beside Sayyiduna Anas bin Malik (may Allah be pleased with him), and he said to me: "Do you know why this piece of wood has been placed here?" I said: "By Allah, I do not know." He said: "the Messenger of Allah (peace and blessings be upon him) used to place his hand on it and say: 'Istawu, i'dilu sufufakum' (Be level (straighten up), straighten your rows).'"
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2294