Explanation of what came in the Sunnah regarding how the one who is praying returns the greeting.

The Shaykh : That I should dwell on this matter a bit is only because it is a greeting that many people have neglected today. It takes two forms: one relates to sending blessings upon the one who is praying, and the other relates to returning the greeting when leaving the assembly. As for the first issue, which is offering the greeting to the one standing in prayer or to those standing in prayer, this is a recommended practice (sunnah) that has been abandoned in both knowledge and practice. Therefore, I wanted to mention it, for the reminder benefits the believers.
Among the spreading of the greeting is what is established from the Prophet (peace and blessings be upon him) that he visited the Ansar one day in their mosque, the Mosque of Quba. So whenever a group entered to greet the Prophet (peace and blessings be upon him) while he was standing in prayer and they offered the greeting to him, he returned the greeting to them with a gesture of his hand. The narrator of the hadith, Ja'far ibn Barqan, said: This gesture is the return of the greeting by the one occupied in prayer to the Muslim who greeted him verbally. Since the one praying was occupied with what he was doing, such as reciting the Qur'an or the tasbih appropriate for each pillar, he was unable to return the greeting verbally, even though that was prescribed (mashru') at the beginning of Islam. It was reported from Ibn Mas'ud (may Allah be pleased with him) that when he returned from the migration to Abyssinia, the first thing he encountered was the Prophet (peace and blessings be upon him) in Madinah, and he found him praying. So he offered the greeting to him verbally, as is obligatory. But he (may Allah be pleased with him) was surprised by what he had not expected, which was that the Prophet (peace and blessings be upon him) did not return the greeting to him verbally, but rather gestured to him with his head like this. Abdullah ibn Mas'ud said: "I was seized by what was near and what was far" He was thinking to himself, questioning: What have I committed? What have I done that the Prophet (peace and blessings be upon him) did not return my greeting? When the Messenger of Allah (peace and blessings be upon him) finished his prayer, he said: So, this speech between the Muslim greeting the one praying and the one praying returning the greeting was verbally like this. Abdullah parted from his Prophet (peace and blessings be upon him) when he migrated to Abyssinia. Therefore, it was a surprise to him what he saw from the Prophet (peace and blessings be upon him) for the first time after he had parted from him, because he had known that the Prophet (peace and blessings be upon him) returned the greeting in prayer verbally. That is because speech was among the rulings that were not immediately forbidden in prayer; that is, it was not among the things forbidden in prayer when they were commanded to pray. Rather, speech was forbidden later. So much so that the one praying would enter the mosque and find the prayer already established, and he would not know if this was the first or second rak'ah. He would stand in the row and ask the one standing in prayer which rak'ah this was? This was before the prohibition of speech in prayer. He would answer him, "This is the first rak'ah" or "This is the second rak'ah." If he had missed a rak'ah, what would he do? This one who missed a rak'ah would come with the missed rak'ah and then join the imam in his prayer. This was the case at the beginning, before the prohibition of speech in prayer. Until one day Mu'adh ibn Jabal (may Allah be pleased with him) entered and did not do as they used to do. He did not ask the one next to him, but rather entered the state of ihram and followed the Prophet (peace and blessings be upon him). Then, when the prayer was finished, he stood up and performed the missed rak'ah. He (peace and blessings be upon him) said: Meaning, there was no longer a need for the one who joined late to perform the missed rak'ah as they used to do before, after asking the praying people, but rather he should do as Mu'adh (may Allah be pleased with him) did. And this is what he (peace and blessings be upon him) affirmed in the hadith agreed upon by the two Shaykhs, al-Bukhari and Muslim, where they narrated from the Prophet (peace and blessings be upon him): Meaning, as Mu'adh (may Allah be pleased with him) did. And Allah the Exalted and Majestic revealed the abrogation of the previous ruling, which was the conversation of a man with his brother in prayer. So Allah revealed His saying, Blessed and Exalted is He: Meaning: silent, no speech in prayer.
It appears that some of the Companions who did not grasp this new ruling, which is preventing the one praying from speaking in prayer, remained with their old knowledge that we mentioned earlier, with which Ibn Mas'ud parted from the Messenger (peace and blessings be upon him). Therefore, when he offered the greeting, he hoped to hear the return of the greeting from his (peace and blessings be upon him) mouth. And you know the rest of the hadith.