Ablution is a prerequisite to prayer, so it is not accepted without ablution. Therefore, Islamic law detailed the matters invalidating ablution. This hadith indicates the lawfulness of sleeping before prayer as long as one keeps his state of ablution. Anas ibn Malik said, "The Prophet's companions (ﷺ) used to doze off then offer prayer without repeating ablution." They used to do so in the presence of the Prophet (ﷺ) who did not decline their behavior. In a narration of Abu Daoud, Anas said, "until their heads lowered down." This means that their heads lowered down due to dozing while they were waiting for prayer. His saying, "... without repeating ablution." is a confirmation that they did not repeat their ablution, for while they were dozing, they were aware of what was around them. They did not sleep deeply, which invalidates ablution, unlike the heavy sleeping that, of course, invalidates ablution due to the lack of awareness..