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, let him step forward himself and fill it. If he does not do so and someone passes by, let him step over his neck, for he has no sanctity."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 11138