Fiqh-us-Sunnah Volume 2, Supererogatory Prayer, Fiqh 2.010A.
Section : Non-stressed sunnah prayers (As-sunan Ghair al-mu’akkadah).
We have been discussing the sunnah prayers which were stressed by the Prophet and which he was careful not to miss. There are some other sunnah prayers (al-sunan ar-ratibah) which are commendable, but are not “stressed.”