The scholars differed concerning acting upon weak ahaadeeth which encourage righteous deeds. Some of them were of the view that it is permissible to act upon them, subject to certain conditions, and others were of the view that it is not permissible to act upon them. 

Al-Haafiz ibn Hajar (may Allaah have mercy on him) summed up the conditions for it to be permissible to act upon a saheeh hadeeth, which are as follows: 

1 – It should not be very weak, and one should not act upon a hadeeth which was narrated only by one of the liars or those who are accused of lying, or whose mistakes are serious. 

2 – It should mention a good deed for which there is a basis in sharee’ah. 

3 – When acting upon it one should not believe that the action is well-founded, rather he should do it on the basis of erring on the side of caution. 

Acting upon a weak hadeeth does not mean that we believe it is mustahabb to do an act of worship simply because a da’eef hadeeth has been narrated concerning it. None of the scholars has said such a thing – as we shall see from the words of Shaykh al-Islam Ibn Taymiyah, below – rather what it means is that if it is proven that a certain act of worship is mustahabb because there is sound (saheeh) shar’i evidence – as in the case of qiyaam al-layl (supererogatory prayers at night), for example – then we find a da’eef hadeeth which speaks of the virtue of qiyaam al-layl, then there is nothing wrong with acting upon this weak hadeeth in that case. What is meant by acting upon it is narrating it in order to encourage people to do this act of worship, in the hope that the one who does it will earn the reward mentioned in the da’eef hadeeth, because acting on the weak hadeeth in this case will not lead to doing something that is forbidden in sharee’ah, such as saying that an act of worship is mustahabb that is not proven in sharee’ah. Rather, if he earns this reward all well and good, otherwise no harm is done.  

Shaykh al-Islam Ibn Taymiyah (may Allaah have mercy on him) said in Majmoo’ al-Fataawa, 1/250: 

It is not permissible in Islam to rely on weak ahaadeeth that are neither saheeh nor hasan, but Ahmad ibn Hanbal and other scholars regarded it is permissible to narrate reports concerning righteous deeds which are not known to be proven, so long as they are not known to be lies, on the basis that if an action is known to be prescribed in Islam from shar’i evidence, and there is a hadeeth which is not known to be a lie, it is possible that the reward referred to in that weak hadeeth may be true. None of the imams said that it is permissible to regard something as obligatory or mustahabb on the basis of a weak hadeeth; whoever says that is going against scholarly consensus. It is permissible to narrate reports that are not known to be lies in order to encourage and warn people, but only with regard to matters where it is known that Allaah has encouraged or warned against them on the basis of other evidence the status of whose narrators is not unknown. End quote. 

Abu Bakr ibn al-‘Arabi said that it is not permissible to act on the basis of a weak hadeeth at all, whether with regard to virtuous deeds or otherwise… See Tadreeb al-Raawi, 1/252. 

This is the view favoured by al-Albaani (may Allaah have mercy on him). See the introduction to Saheeh al-Targheeb wa’l-Tarheeb, 1/47-67. 

The saheeh proven reports from the Prophet (peace and blessings of Allaah be upon him) offer us sufficient evidence that we have no need to act on the basis of weak hadeeths. 

The Muslim must strive to find out which ahaadeeth are sound (saheeh) and which are weak (da’eef), and be content to act on the basis of the sound reports. 

And Allaah knows best.