Should we place green leaves on graves so that the punishment of the deceased may be reduced?
Question

I heard in a hadeeth that the Prophet (peace and blessings of Allah be upon him) placed two leaves on two graves whose occupants were being tormented, so that the punishment might be reduced, but I do not understand the reason for that. Is it prescribed for us to do likewise?.
Praise be to Allah.Firstly: 
Yes, it is proven that the Prophet (peace and blessings of Allah be upon him) passed by two graves and said: “They are being punished but they are not being punished for something that was difficult to avoid [?}.” Then he said, “One of them used to go about spreading malicious gossip, and the other used not to take precautions to avoid being contaminated with urine.” Then he took a fresh branch, broke it in two and planted each piece on a grave. Then he said: “May their punishment be reduced so long as this does not dry out.” 
Narrated by al-Bukhaari, 1378; Muslim, 292. 
This indicates that the punishment may be reduced, but what is the relationship between these two branches and the reduction of these punishments for these two persons? 
1 – It was said that the branches glorify Allah (by saying “Subhaan-Allah” i.e., tasbeeh) so long as they do not dry out, and tasbeeh reduces the punishment of the deceased. Based on this they reached the conclusion – which far-fetched – that it is Sunnah for a person to go to the grave and recite tasbeeh so that the punishment of the deceased might be reduced. 
2 – Some of the scholars said: This reason is da’eef (weak) because the two branches would recite tasbeeh whether they were fresh or dry, because Allah says (interpretation of the meaning): 
“The seven heavens and the earth and all that is therein, glorify Him and there is not a thing but glorifies His Praise. But you understand not their glorification”
[al-Isra’ 17:44]
Pebbles in the hands of the Messenger (peace and blessings of Allah be upon him) were heard to recite tasbeeh even though pebbles are dry, so how are we to understand the hadeeth? 
We may understand it as meaning that the Messenger (peace and blessings of Allah be upon him) hoped that Allah would reduce their punishment so long as these two branches remained fresh, so it was not a long period. This was so as to warn against doing what they did, because their actions were serious as it says in the report that the Prophet (peace and blessings of Allah be upon him) said, “But it is serious.” 
One of them did not take precautions to avoid being contaminated by urine, and if he was not free of such contamination then he was praying without being pure. 
The other used to walk about spreading malicious gossip, causing mischief among the slaves of Allah and stirring up enmity and hatred among them. This is a serious matter. 
This is the most likely understanding of the meaning of the hadeeth, that it was a temporary intercession and a warning to the ummah, not because the Messenger (peace and blessings of Allah be upon him) wanted to offer ongoing intercession.  
Secondly: 
Some scholars – may Allah forgive them – say that it is Sunnah to place a fresh branch, etc on the grave so that the punishment might be reduced. But this understanding is very far-fetched, and it is not permissible for us to do that for several reasons. 
(i) We do not know that this man is being punished, unlike the Prophet (peace and blessings of Allah be upon him) whom Allah told by means of Revelation about the situation of those two graves.
(ii) If we do that, we are mistreating the deceased, because we are thinking badly of him and assuming that he is being punished, but we do not know that. It may be that he is being blessed, or it may be that this deceased person is one of those whom Allah blessed with forgiveness before he died because of one of the many things that bring forgiveness, so he died having been forgiven by the Lord of all people, in which case he would not deserve to be punished.
(iii) This understanding is contrary to the way of the righteous salaf. This action was not part of their way and they were the most knowledgeable of people about the sharee’ah of Allah.
(iv) Allah has shown us something that is better than that. When the Prophet (peace and blessings of Allah be upon him) finished burying someone he would stand over the grave and say, “Pray for forgiveness for your brother and ask that he be made steadfast, for even now is he being questioned.”
From Majmoo’ Fataawa al-Shaykh Muhammad ibn ‘Uthaymeen (may Allah have mercy on him), 2/30.