Surah 48

Surah 48:4

"He (it is) who sent down the Sakīna into the hearts of the believers, so that they might add belief to their belief – to God (belong) the forces of the heavens and the earth, and God is knowing, wise –"

48.4 – He (it is) who sent down the Sakīna into the hearts of the believers

The Arabic word Sakīna appears three times in this sūra: Allah sends down Sakīna on his messenger (v. 26) and on the “believers” (vv. 4, 18, 26). See the comment on Sakīna at 2.248.

- from The Quran with Christian Commentary: A Guide to Understanding the Scripture of Islam