All perfect praise be to Allah the Lord of the Worlds. May His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
It is mentioned in Hadiths that it is recommended for those attending Friday prayer to listen attentively to the Khateeb (One who delivers Friday sermon) and that it is forbidden for them to engage in any idle activity. Prophet Muhammad (peace be upon him) said, “He who touches pebbles, [during the Friday sermon] has engaged in idleness.” (Muslim). Interpreting this Hadith, Judge Eyad said: "By touching the pebbles, he has engaged in idle activity and distracted other prayer performers." {Ekmal al-Mo`alim, V.3:253}." "It refers to listening attentively to the Khateeb with one`s mind and heart." {Shareh An-Nawawi al Muslim, V.6:147}.
In order to attain the full reward for attending the Friday prayer, it is recommended to listen to the Khateeb attentively and not engage in any idle activity, such as talking or using cell phone.
Scholars have disagreed about the meaning of (He who touches pebbles, [during the Friday sermon] has engaged in idleness). Shafie jurists said that it indicates reprehensibility. "It is disliked for a person to talk to another during Friday sermon based on the Hadith reported by Muslim:" On Friday, when the Imam is delivering the Khutbah (religious talk), if you say to your companion to keep quiet, then indeed you have committed a sin (error/something senseless)." This Hadith means that you haven`t observed the etiquettes of Friday prayer." {Nehayat al-Mohtaj, V.2:321}. According to Al-Mawardi and other scholars, it is unanimously agreed that engaging in voluntary prayer after the Khateeb ascends the pulpit to deliver the Friday sermon is an abominable matter that is almost unlawful.
Given that it is disliked to touch the pebbles during the Friday sermon, then using cell phone (WhatsApp, Facebook) is far more distracting for the person himself and for the others. Moreover, given that it is forbidden to deliver voluntary prayers during the Friday sermon, then using the cell phone is far more forbidden.
In conclusion, a Muslim must avoid engaging in any idle activity during the Friday sermon in order to receive the full reward from Allah the Almighty.
