Does this Du‘aa’ (supplication): "May Allaah elongate your lifespan" change the lifespan of a person? Does a person’s lifespan increase and decrease? What is the explanation of the Hadeeth, "O Allaah, make me enjoy my husband, the Messenger of Allaah"? I think this Hadeeth indicates the increase of the lifespan.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu 'alyhi wa sallam, is His slave and Messenger.

The aforementioned Hadeeth was narrated on the authority of Ibn ‘Abbaas and his father, where he said that Umm Habeebah, the Prophet's wife asked Allaah The Almighty, to make her enjoy her husband - the Messenger of Allaah, sallallaahu ‘alayhi wa sallam, her father - Abu Sufyaan, and her brother - Mu‘aawiyah - . The Messenger of Allaah, sallallaahu ‘alayhi wa sallam, told her that as such she had asked for a specific lifespan and certain gifts from Allaah The Almighty and that all these are matters that have already been decreed in advance by Allaah The Almighty. He also told her that everything will occur at the time designated by Allaah The Almighty; not later and not earlier. [Muslim]

This Hadeeth clearly indicates that life-spans, livelihood and other matters will take place according to what has been decreed in advance by Allaah The Almighty, and that there will be no increase or decrease in such things.

There are other Hadeeths that some people may find contradict this Hadeeth. For example, in a Hadeeth on the authority of Anas ibn Maalik the Messenger of Allaah, sallallaahu ‘alayhi wa sallam, said that maintaining good relations with one's kindred lengthens one's lifespan and increases one’s livelihood. [Muslim]

Moreover, in a Hadeeth on the authority of Salmaan the Messenger of Allaah, sallallaahu ‘alayhi wa sallam, said that only Du‘aa’ (supplication) changes Fate, and that only kind treatment lengthens one’s lifespan. [At-Tirmithi:  Hasan (Sound)]

Scholars hold different opinions when seeking to reconcile these Hadeeths:

An-Nawawi reconciled these texts as follows:

1- This increase means that Allaah The Almighty blesses one's lifespan, and guides one to do good acts and to occupy one's time in doing good things that will benefit one in the Hereafter. 

2- Angels read in Al-Lawh Al-Mahfooth (The Preserved Tablet) for example, that a person's lifespan will be sixty years, unless he maintains good relations with his kindred. If he maintains good relations with his kindred, his age would then be 100 and Allaah The Almighty knows, of course, in advance what will happen. He Says (what means): {Allaah eliminates what He wills or confirms.} [Quran 13:39] 

Therefore, no increase takes place in relation to the knowledge of Allaah The Almighty and in relation to what He had decreed in advance. This increase may take place in relation to what the angels know as they follow what is written in Al-Lawh Al-Mahfooth.  This is what is meant by this Hadeeth.

3- This increase means that people would mention the merits of a person after his death. Therefore, this person’s memory is kept  alive. Al-Qaadhi mentioned this opinion, yet it is incorrect. This is understood from the words of An-Nawawi .

Reconciling these texts, At-Tahaawi said that when Allaah The Almighty wants to create a person, He may decree that his lifespan would stretch to be such and such an amount of time if he treats his parents kindly.  Whereas his lifespan would stretch to be such and such a time if he does not treat his parents kindly. Moreover, Allaah The Almighty may decree that if this person supplicates to Him, he would not face such and such (an event), whereas if he does not supplicate to Him, he would face such and such an event. Furthermore, Allaah The Almighty may decree that if he does such and such, he would be deprived of such and such, whereas if he does not do such and such, he would be given such and such. Allaah The Almighty has recorded all these decrees in Al-Lawh Al-Mahfooth, to which nothing can be added nor can anything be removed from. This is understood from the words of At-Tahaawi .

As such these texts can be reconciled, and are not contradictory, and all perfect praise is due to Allaah The Almighty. 

Allaah Knows best.