Hadith: During the Prophet's lifetime, the Companions used to wait for the ‘Ishā' prayer so much so their heads were lowered down (by dozing). They would then pray without performing (a new) ablution

عن أنس بن مالك رضي الله عنه قال: كان أصحاب رسول الله صلى الله عليه وسلم على عَهدِه ينتظرون العشاء حتى تَخْفِقَ رُءُوسُهُمْ، ثم يصلُّون ولا يَتَوَضَّئُونَ.
[صحيح] - [رواه مسلم وأبو داود واللفظ له]

Anas ibn Mālik (may Allah be pleased with him) reported that during the Prophet's lifetime, the Companions used to wait for the ‘Ishā' prayer so much so their heads were lowered down (by dozing). They would then pray without performing (a new) ablution.
Sahih/Authentic. - [Abu Dawood]

Explanation

The Companions during the time of the Prophet (may Allah's peace and blessings be upon him) used to wait for the ‘Ishā' prayer, and they would doze off then pray without making ablution again. In principle, anyone does an act during the time of the Prophet (may Allah's peace and blessings be upon him) and the Prophet did not denounce it, this is considered a tacit approval. The Prophet’s approval of an action is part of the Sunnah, which generally includes his sayings, actions, and approvals. Any act done during the Prophet's lifetime and he did not denounce it, then it is considered an act of Sunnah by approval. Had their prayers been invalid or their action been incorrect, the Prophet (may Allah's peace and blessings be upon him) would have informed them of it once it comes to his knowledge or is conveyed to him through the revelation. Unlike the actions that occur after the Prophet’s death. The Companions' heads would be dropping from drowsiness to the extent that the narrator, according to another version, could hear some of them snoring or see them laying on their sides, then they would get up and pray without making ablution. This means that they would get up to pray without making a new ablution, because their sleep was not deep. Had such sleep invalidated their ablution, the Prophet would not have approved that they pray without making a new ablution. By this understanding, one can reconcile between the other textual proofs, since it was authentically reported that sleeping invalidates ablution, exactly like defecation and urination. The Prophet (may Allah's peace and blessings be upon him) said regarding invalidators of ablution (which do not require removal of leather socks): "...not for defecation, urination, or sleep." Ali,(may Allah be pleased with him) reported that he said: "The eye is the string tying the anus, so anyone sleeps let him make ablution." Mu‘āwiyah (may Allah be pleased with him) reported that he said: "The eye is the string tying the anus, so if the eyes sleep, the string is untied (i.e. wind is free to pass out)." All these narrations prove that sleeping renders ablution invalid, whereas the Hadīth mentioned here and the associated narrations indicate that sleeping does not invalidate ablution. Sleeping, therefore, in the Hadīth mentioned here as well as the narrations that mention "snoring" and "lying down on their sides" should be interpreted to mean that it is not deep. In reality, one may hear someone snoring during the initial stage of sleeping and before falling into deep sleep. Also, lying down on one’s side does not necessitate that a person is in deep sleep. So, all the texts based on this interpretation are in complete agreement and employed. In principle, no evidence should be discarded as long as they could be reconciled and applied together. In brief, if a person goes into deep sleep such that he completely loses consciousness, then this type of sleep requires ablution. If the sleep is not deep, then it does not require ablution although renewing ablution is preferable and safer to ensure the validity of prayer. If one is doubtful whether he was sleeping deeply or not, then his ablution does not become invalid. This is because the default is that he is certain about being in a state of ritual purity, and such certainty cannot be removed by doubt. Shaykh al-Islam Ibn Taymiyyah (may Allah have mercy upon him) said: "Sleep during which one is doubtful whether he passed wind or not, does not invalidate the ablution. This because the state of purity was established with certainty, so it cannot be removed by doubt."