Mu‘āwiyah ibn Abu Sufyān (may Allah be pleased with him) reported that the Messenger of Allah (may Allah's peace and blessings be upon him) said: "Verily, the two eyes (when awake) are the string that ties up the anus, but when the two eyes sleep, the string is untied."
Meaning of the Hadīth: "The two eyes are the string that ties up the anus", i.e. the eyes in the state of wakefulness can control the anus, and prevent anything from being discharged therefrom. If something comes out from it, one will feel it. "But when the two eyes sleep, the string is untied ", i.e. when one goes to sleep, his body muscles relax, so the string (or muscle) that controls his anus becomes loose and wind can pass freely without him feeling it. The Prophet (may Allah's peace and blessings be upon him) likened the eyes to the string that is tied on top of a bag. If the two eyes are opened, then the string will be tight around the anus, so if something comes out, one will feel it. However, if the eyes are asleep, then the string will be untied and everything inside the bag will come out without one feeling it. This is a type of simile. It is an eloquent simile which the Prophet (may Allah's peace and blessings be upon him) used to make the Shariah ruling more understandable. It is an example of the comprehensive precise speech which the Prophet (may Allah's peace and blessings be upon him) was endowed with.