Fiqh-us-Sunnah Volume 4, Funerals and Dhikr, Fiqh 4.123A.

Section : On Seeing Another Muslim Wearing Some New Clothes.

According to a sound hadith, the Prophet, peace be upon him, dressed Umm Khalid, a small girl, in a black woollen blanket and said to her, “May you live so long that you will wear out many garments.” The Companions used to say, “May you live so long that you wear out this garment and may Allah replace it with a better one.” On seeing ‘Umar wearing some new clothes the Prophet, peace be upon him, said, “May you wear new clothes and may you live nobly and die as a happy martyr.’ (Reported by Ibn Majah and Ibn As-Sinni)