It is narrated from the wife of the Prophet, Mother of the Believers, Sayyida Hafsa (may Allah be pleased with her), she says that the Messenger of Allah (peace and blessings be upon him) would (at times) perform voluntary prayers while sitting. He would recite the surah slowly and with pauses, until that surah would become longer than a surah that was originally longer than it.
Hadith Referenceشمائل ترمذي / باب ما جاء فى عبادة رسول الله صلى الله عليه وسلم / 280
Hadith Gradingزبیر علی زئی:صحيح
Hadith Takhrij{ « صحيح » } : «(سنن ترمذي : 373 ، وقال : حسن صحيح) ، صحيح مسلم (733) ، موطأ امام مالك (رواية يحييٰ 137/1 ح 307 ، رواية ابن القاسم : 7)»