The Prophet ﷺ said, "There is to be no diminishment in prayer or in the greeting of peace."Ahmad said, "What I understand by this is that you should not give the greeting of peace during prayer, nor should anyone give it to you, and that a man should not shorten his prayer and depart from it while still in doubt about it."
عن النبي ﷺ، قال: لا غرار في صلاة ولا تسليم قال أحمد: يعني -فيما أرى- أن لا تسلم ولا يسلم عليك. ويغرر الرجل بصلاته فينصرف وهو فيها شاك.
Abu Dawud (928) narrated it from Ahmad bin Hanbal, who said, "Abdul-Rahman bin Mahdi narrated to us, from Sufyan, from Abu Malik Ashjai, from Abu Hazim, from Abu Hurairah," and he mentioned it. This is a sahih isnad. Imam Ahmad narrated it in his Musnad (9936), and through his route Hakim also narrated it (1/264) and said, "Sahih according to the standard of Muslim." I say, and that is indeed the case, for Abu Malik Ashjai's name is Saad bin Tariq, and he is among the men of Muslim.However, Abu Dawud pointed out a flaw, saying, "Ibn Fudayl narrated it with the same wording as Ibn Mahdi, but did not raise it to the Prophet." I say, Ibn Fudayl is Muhammad bin Fudayl bin Ghazwan, whom some critics have discussed, saying he cannot be relied upon as sole authority, though he was truthful. Abdul-Rahman bin Mahdi is more reliable and has stronger memory than him, so his disagreement does not harm the hadith, for an addition from a trustworthy narrator is accepted.Likewise, the hadith is not to be faulted on account of the doubt expressed by Muawiya bin Hisham, who narrated it from Sufyan, from Abu Malik, from Abu Hazim, from Abu Hurairah, saying, "I think he raised it to the Prophet," and then he mentioned the hadith. Abu Dawud (929) narrated it from Muhammad bin Ala, from Muawiya bin Hisham, with this wording. For certainty is not nullified by doubt.As for the meaning of his statement, "There is no ghirar in prayer," it has two interpretations. The first is that one should not perform his bowing or his prostration incompletely. The second is that if one is in doubt whether he has prayed three or four rakats, he should go with the greater number, setting aside certainty and ending the prayer based on mere doubt.The sunnah has come, in the narration of Abu Saeed Khudri, that one should disregard the doubt and build upon what is certain, praying an additional rakat until he knows that he has completed four. This is what Khattabi said.As for his statement, "and no tasleem," its meaning, as Imam Ahmad said, is that neither should you greet anyone nor should anyone greet you, meaning it is not permissible to speak during prayer with speech unrelated to it.