Question Summary:
Reciting Surah Yaseen at a graveyard for 40 days.

Question Detail:

One of my friend ask me, "is reciting surah yasin at graveyard cause the waving of punishment in grave ( Azabul Kobar) for 40 days."
 He heard  it does He want to know is there any authentic hadith in support of this.

Answer :

In the Name of Allah, the Most Gracious, the Most Merciful.
As-salāmu ‘alaykum wa-rahmatullāhi wa-barakātuh.
 
It is stated in the following hadith:
" عن أنس ^رض أن رسول الله صلى الله عليه وسلم قال: من دخل المقابر فقرأ سورة يس خفف الله عنهم و كان له بعدد من فيها حسنات. " أخرجه عبد العزيز صاحب الخلال، كما في شرح الصدور و المظهري و أبو بكر النجار كما في معدة القاري... علامہ کبير حافظ الدين سيوطي ان احاديث و آثار کے باے میں فرماتے ہیں: و هي و إن كانت ضعيفة فمجموعها يدل على أن لذلك أصلا.
Translation: Anas, radiyllahu, narrated that the Prophet, sallallahu alayhi wa sallam, said, “Whoever enters a graveyard and reads surah Yasin, Allah will lessen the punishment of the deceased and the reciter’s reward will be according to the number of people buried there.”[1] 
The hadith is considered reliable. The decreasing of the punishment from the inmates of the graves upon reading Surah Yaseen is general.
There is no mention of the punishment in the grave being waived for 40 days.
And Allah Ta’āla Knows Best
Muhammad Haris Siddiqui
Student Darul Iftaa
Australia
Checked and Approved by,
Mufti Ebrahim Desai.