Satta the drummer and Rai Balwand the rebeck player were the court musicians of the early Gurus. Their composition, the Ramkali ki Var, is in the Guru Granth Sahib Ji.
It is a history in verse of the succession from Guru Nanak Dev Ji to Guru Arjan Dev Ji, arguing that the light passed unchanged from body to body. It is the earliest Sikh account of the succession and it was written by employees rather than by disciples.
It also names Mata Khivi Ji, wife of Guru Angad Dev Ji, for the langar she ran, describing the shade she gave and the rich food she served. She is one of the very few women named in the volume and she is named for feeding people.
The tradition records that the two fell out with Guru Arjan Dev Ji over payment, refused to sing, were replaced, and were later reconciled. Their composition was kept.
The Var is sung in a specific tune to this day and is one of the fixed points of the kirtan repertoire.
They were rababis, the hereditary musicians attached to the Guru's establishment, and the vaar they composed is in the Guru Granth Sahib Ji.
The Ramkali vaar praises the first five Gurus in sequence and names Mata Khivi Ji and the langar she ran, which makes it the earliest historical source in the volume.
The tradition also records that they quarrelled with Guru Arjan Dev Ji and were reconciled. That the quarrel is preserved along with the composition says something about how the volume was assembled.