AsSalamu Alykum . What is the sunnah of breast feeding ?
Is breast feeding while lying down is against sunnah?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

We have not come across any particular position that is Islamically required from a mother when breast-feeding her child; so there is flexibility in the matter. Thus it is permissible for a woman to breastfeed her child while standing, sitting, or lying down; she is permitted to breastfeed in any of these positions.

Allaah Knows best.