He asked Abdullah bin Umar about the prayer of the Messenger of Allah ﷺ, and he replied, "He would say 'Allahu Akbar' every time he went down and 'Allahu Akbar' every time he came up. Then he would say, 'Peace and the mercy of Allah be upon you,' to his right, and 'Peace and the mercy of Allah be upon you,' to his left."
أنه سأل عبد الله بن عمر عن صلاة رسول الله ﷺ فقال: الله أكبر كلما وضع، الله أكبر كلما رفع، ثم يقول: السلام عليكم ورحمة الله عن يمينه السلام عليكم ورحمة الله عن يساره.
Nasai (1320) narrated it. He said, Hasan bin Muhammad Zafarani informed us, from Hajjaj, who said, Ibn Jurayj told us, Amr bin Yahya informed us, from Muhammad bin Yahya bin Habban, from his uncle Wasi bin Habban, that he asked Ibn Umar, and he mentioned the same. Its isnad is sahih, and Ibn Khuzayma (576) graded it sahih as well, narrating it also from Hasan bin Muhammad. Ibn Abdul-Barr graded it hasan in Tamhid (16/189). Ibn Jurayj is a mudallis, but he stated clearly that he heard it directly. Hajjaj is Ibn Muhammad Masissi, one of the men of the Jamaah.This is not undermined by what Shafii narrated in Umm (1/122) from Muslim bin Khalid and Abdul-Majid, from Ibn Jurayj, with the same wording. Muslim bin Khalid, known as Zanji, and Abdul-Majid, who is Ibn Abdul-Aziz bin Abu Rawwad, have both been criticized, though they are truthful narrators who make mistakes. For this reason Bayhaqi said in Marifah (3844), after narrating it through Shafii's route, "Hajjaj bin Muhammad likewise narrated it from Ibn Jurayj."However, Darawardi narrated it from Amr bin Yahya Mazini, from Muhammad bin Yahya bin Habban, from his uncle Wasi, saying once "from Ibn Umar" and once "from Abdullah bin Zayd," that the Prophet ﷺ used to say the salam to his right and to his left. Shafii narrated this, and Bayhaqi narrated it through his route in Marifah (3846). This is how Shafii narrated it from him, with uncertainty. But Imam Ahmad (5402) narrated it from Abu Salama, and Nasai (1321) narrated it from Qutayba bin Saeed, both of them from Abdul-Aziz Darawardi, from Ibn Umar, without any uncertainty. Ahmad's wording mentions the takbir every time he lowered his head and every time he raised it, and mentions "Peace be upon you and the mercy of Allah" to his right, and "Peace be upon you" to his left.What appears to be the case is that Darawardi used to narrate it through two different routes, or else he fell into error due to a weakness in his memory, in which case the correct view is that it is from the hadith of Ibn Umar, given the soundness of Ibn Jurayj's narration through his route.