Those who did not hold greeting one who is praying

Abu Sufyan said that Sayyiduna Jabir bin Abdullah (may Allah be pleased with them both) said: "If I were to come upon people who are praying, I would not greet them with salam."

(a) It is narrated from Abu Hurayrah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "There should be no deficiency (ghirar) of any kind in the prayer or in replying to a greeting."
(b) Imam Ahmad bin Hanbal (may Allah have mercy on him) said: Ghirar regarding the salam means that, while in prayer, one should neither greet anyone nor should anyone greet you; and ghirar regarding the prayer is that a man finishes his prayer and then falls into doubt whether he prayed three rak'ahs or four. This is how it is in my book.
(c) The same wording is also reported through another chain: there should be no slackness in the prayer or in replying to a greeting.
(d) Imam Ahmad bin Hanbal (may Allah have mercy on him) said: I think what he (peace and blessings be upon him) meant is that during the prayer you should neither greet anyone nor should anyone greet you, and ghirar in the prayer means that a man finishes the prayer and gives the salam while he is in doubt about the prayer. This is in accordance with the explanation of Ahmad bin Hanbal (may Allah have mercy on him).
(e) Imam Abu Dawud (may Allah have mercy on him) said: Ibn Fudayl narrated it from Abu Malik in the wording of Ibn Mahdi, and did not attribute it to the Prophet (marfu').

Mu'awiyah bin Hisham narrated it from Sufyan with his chain; he said: I think he attributed it to the Prophet: "There is no deficiency (ghirar) in the salam or in the prayer."
This wording negates slackness in both the salam and the prayer. The earlier hadiths make it permissible to greet someone who is praying and for the one praying to reply with a gesture, and they are more worthy of being followed. Wa billahi al-tawfiq (And success comes only from Allah).