This hadith is about the importance of praying in congregation. It was narrated by Sayyar bin al-Ma'roor that he heard 'Umar giving a khutbah, and he said that the Messenger of Allah built this mosque and we, the Muhajireen and Ansar, were with him. If it gets too crowded, then let one of you prostrate on his brother's back. On one occasion, Prophet Muhammad saw people praying in the street and said to them to pray in the mosque instead. This hadith teaches us that we should always try to attend congregational prayers at mosques as much as possible because it is a great way for Muslims to come together as a community and strengthen their faith through prayer. It also shows us how important it is for us to look out for each other’s well-being even when we are worshipping God together - if there isn't enough space for everyone inside the mosque then those who can should offer their backs so others can prostrate on them during prayer time.