It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "Whoever sees a gap in a row should fill it himself. If he does not do so, and someone passes by, then let that person step over his neck, for he has no sanctity."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 11168