The treatise by Imam ibn Hanbal, the Imam of the Hanbali school of juristic reasoning, renowned for his steep knowledge, was written several hundred years ago to the inhabitants of a town where the Imam stayed for a period of time. It contains a full and clear description of the prayer and includes detailed observations of the mistakes, which Imam Ahmad observed during his time in that town. It was and remains an invaluable work for all Muslims. Detailing as it does many common errors made during prayers, some of which are serious enough to invalidate the act of worship.
