Jābir ibn ‘Abdullāh (may Allah be pleased with him) reported that a man came while the Prophet (may Allah's peace and blessings be upon him) was giving the Friday sermon. So he (the Prophet) said to him: "O So and so, have you prayed?" He said: 'No.' The Prophet (may Allah's peace and blessings be upon him) said: "Get up and pray two Rak‘ahs." In another narration: "Then pray two Rak‘ahs."
Sulayk al-Ghatafāni entered the Prophet's mosque while the Prophet (may Allah's peace and blessings be upon him) was giving the sermon on Friday. He sat down to listen to the sermon without performing the two-Rak‘ah mosque-greeting prayer. He was either unfamiliar with the ruling on that payer or thought that listening to the sermon was more important than performing it. The Prophet's engagement in giving the sermon did not prevent him from teaching Sulayk; rather, he addressed him asking if he had prayed in the corner of the mosque before he spotted him. When Sulayk said that he had not prayed, the Prophet commanded him to get up and pray two Rak‘ahs. According to a report in Sahīh Muslim, the Prophet asked him to pray two short Rak‘ahs. The Prophet gave instructions to the man in the presence of a big gathering for two reasons: to teach him something that he needed to learn at the time of need and to educate others as well. So for anyone entering the mosque while the Imām is giving the sermon, it is prescribed for him to pray before sitting to listen to it. This Hadīth is evidence for it, along with another Hadīth that reads: "If one of you comes to the mosque on Friday while the Imām is giving the sermon, let him pray two Rak‘ahs and keep them short." Commenting on this Hadīth in his explanation of Sahīh Muslim, An-Nawawi said: "This is an explicit text that may not be subjected to interpretation. I do not think that a scholar who is informed of this wording and believes in its authenticity would go against it."