From Sahih Bukhari (Hadith 4082):
Narrated Khabbab:We migrated with the Prophet (ﷺ) for Allah's Cause, so our reward became due with Allah. Some of us passed away (i.e. died) without enjoying anything from their reward, and one of them was Mus`ab bin `Umair who was killed (i.e. martyred) on the day of Uhud. He did not leave behind except a sheet of striped woolen cloth. If we covered his head with it, his feet became naked, and if we covered his feet with it, his head became naked. The Prophet (ﷺ) said to us, "Cover his head with it and put Idhkhir (i.e. a kind of grass) over his feet," or said, "Put some Idhkhir over his feet." But some of us have got their fruits ripened, and they are collecting them
Reference: Bukhari 4082