(2:131) And “Abraham instructed his sons [Ishmael and Isaac to do the same ... And do not recite your Salah-prayers [too] loudly or [too] quietly, but seek a way in between.” ...

She told Abraham to have a child with her servant Hagar. Hagar and Abraham had a son called Ishmael. This was not what God had promised and later Sarah and Abraham had their own son called Isaac.