In some cases ablution is not mandatory but it is recommended as the person is in his best state when he has performed the ablution. These situations include; reciting Quran, sleeping, or before each prayer.
When ablution is recommended
1-Before glorifying Allah or reciting the Qur’an, because a person is in his best situation when he has performed the ablution.
2-Before every prayer. The Prophet (peace be upon him) always did that,
:as reported by Anas
‘The Prophet (peace be upon him) used to perform the ablution before every prayer.’
Related by al-Bukhari, hadith No. 214
This means that even when he had already performed an ablution that was still valid.
3-A person who is in the state of ceremonial impurity (i.e. janabah) is recommended to perform the ablution if he wants to have a second intercourse, or sleep, or eat or drink.
:Abu Sa'id al-Khudri reported that the Prophet (peace be upon him) said
‘If any of you has had sexual intercourse with his wife and wants a second, he is recommended to perform the ablution first.’
Related by Muslim, hadith No. 308
:A’ishah reported
‘When Allah’s messenger wanted to go to sleep and he was in the state of ceremonial impurity, he would perform the ablution before he went to sleep just as he would do for prayer.’
Related by Muslim, hadith No. 305
:In another version she said
‘When Allah’s messenger wanted to eat or go to sleep.’
Related by Muslim, hadith No. 305
:A’ishah reported
‘When Allah’s messenger wanted to take a bath to remove the state of ceremonial impurity, he would start by washing his hands, and then he would pour water by his right hand over his left to wash his private part, then he would perform the ablution, as he would do for prayer.’
Related by Muslim, hadith No. 316
:Al-Bara’ ibn 'Azib reports that the Prophet (peace be upon him) said
‘When you want to go to bed, perform the ablution as you would for prayer, then lie on your right side.’
Related by al-Bukhari, hadith No. 247; Muslim, hadith No. 2,710