Nor should the Believers all go forth together: if a contingent from every expedition remained behind, they could devote themselves to studies in religion, and admonish the people when they return to them,- that thus they (may learn) to guard themselves (against evil).
Muhammad Shafi Usmani writes,
Shafi, Muhammad (2008). Maariful Quran. (Vol .4 surah 9 verse 122). English-MaarifulQuran-MuftiShafiUsmaniRA-Vol-4.pdf (islamicweblibrary.com)