Sahih Bukhari
Merits of the Helpers in Madinah (Ansaar)
Volume: 005 / Book Number: 058 / Hadith Number: 214
Narrated Um Khalid bint Khalid:
When I came from Ethiopia (to Medina), I was a young girl. Allah's Apostle made me wear a sheet having marks on it. Allah's Apostle was rubbing those marks with his hands saying, "Sanah! Sanah!" (i.e. good, good).