942. Commentary: Isa ibn Ayyub is among the Tabi‘ al-Tabi‘in. Since the purpose in prayer is to alert the imam, therefore using just two fingers is sufficient. Clapping with all the fingers is considered as amusement and play, hence a distinction has been made.
Source: Sunan Abu Dawood – Commentary by Shaykh Umar Farooq Saeedi, Page: 942