Question Summary:
Can you please provide the reference for that Qawl of Rasulullah (Sallallaahu Alayhi Wasallam)?

Question Detail:

Bukhari :: Book 2 :: Volume 13 :: Hadith 4Narrated Abu Said Al-Khudri:
Allah's Apostle (p.b.u.h) said, "The taking of a bath on Friday is compulsory for every male (Muslim) who has attained the age of puberty."

On answering to this Hadith you said : According to all four Madhabs, Ghusl on the day of Jumu?ah is Sunnah, and not Fardh.

The Hadith you are referring to applied during the early period of Islam. Later, Rasulullah (Sallallaahu Alayhi Wasallam) explained that the rule had changed, and that it is no longer Fardh. It, however, remains a Sunnah.

Can you please provide the reference for that Qawl of Rasulullah (Sallallaahu Alayhi Wasallam)?

Answer :

In the name of Allah, Most Gracious, Most Merciful
Assalaamu `alaykum waRahmatullahi Wabarakatuh
It is narrated in Sunan Abi Dawood from Ikrima (رحمه الله) that a group of people from Iraq came to Ibn Abbass (رضي الله عنه) and asked him whether it is obligatory to have a bath on the day of Jumu’ah. He replied: No! (It is not obligatory), however, it is purer and better for the one who has a bath. As for the one who does not have a bath, it is not obligatory upon him.
Thereafter, he explained the condition of the people which prompted the Prophet (صلى الله عليه وسلم) to instruct them to bath on a Friday.
“The people were poor, they used to wear wool and do hard work. The Musjid was small and the roof was low. Once Rasulullah (صلى الله عليه وسلم) came to the Musjid on Friday and it was hot. The people were perspiring in their woollen clothing. An odour emitted which caused harm to those present. When Nabi (صلى الله عليه وسلم) noticed this, he instructed the Sahabah to bath on Friday and to apply the best of fragrances.”
Ibn Abbass (رضي الله عنه) said:  Thereafter, Allah Ta’ala blessed the muslims with much more affluence and they wore clothing other than woollen clothing. The Musjid was expanded and the odour that used to emanate did not exist anymore. (Abu Dawood)
Thus, the bath on the day of Friday is sunnah and not obligatory, as was in the early days of Islam.
 
عن عمرو بن أبي عمرو عن عكرمة أن أناسا من أهل العراق جاءوا فقالوا يا ابن عباس أترى الغسل يوم الجمعة واجبا قال لا ولكنه أطهر وخير لمن اغتسل ومن لم يغتسل فليس عليه بواجب وسأخبركم كيف بدء الغسل كان الناس مجهودين يلبسون الصوف ويعملون على ظهورهم وكان مسجدهم ضيقا مقارب السقف إنما هو عريش فخرج رسول الله صلى الله عليه وسلم في يوم حار وعرق الناس في ذلك الصوف حتى ثارت منهم رياح آذى بذلك بعضهم بعضا فلما وجد رسول الله صلى الله عليه وسلم تلك الريح قال أيها الناس إذا كان هذا اليوم فاغتسلوا وليمس أحدكم أفضل ما يجد من دهنه وطيبه قال ابن عباس ثم جاء الله بالخير ولبسوا غير الصوف وكفوا العمل ووسع مسجدهم وذهب بعض الذي كان يؤذي بعضهم بعضا من العرق
سنن أبى داود  (ج 1 / ص51)
 
And Allah knows best
Wassalaamu `alaykum
Ml. Rayhaan Docrat, Student Darul Iftaa
Concurred by:
Ml. Talha Desai, Student Darul Iftaa
Darul Iftaa, Madrassah In'aamiyyah