➊ By "completing" is meant moderation, tranquility, and recitation of the tasbihat and adhkar, the details of which have already been mentioned in previous ahadith.
➋ The imam should occasionally continue to instruct the congregation regarding the rulings of prayer, especially when the followers are not performing the pillars of prayer correctly.
Source: Sunan Nasa'i: Translation and Benefits by Shaykh Hafiz Muhammad Amin Hafizullah, Page: 1055
´It was narrated that 'Abdur-Rahman bin Al-Asamm said:` "Anas bin Malik was asked about the takbir in the prayer. He said: "The takbir should be said when bowing, when prostrating, when raising one's head from prostration and when standing up following the first two rak'ahs.' Hutaim said: 'From whom did you learn this?' He said: 'From the Prophet (ﷺ), Abu Bakr and 'Umar, may Allah (SWT) be pleased with them.' Then he fell silent and Hutaim said to him: 'And 'Uthman?' He said: 'And 'Uthman.'"
Hafiz Muhammad Ameen
1180. Commentary: The opening takbir (takbir tahrimah) is agreed upon by consensus, and moreover, no one would show any negligence regarding it; therefore, it was not mentioned. As for the remaining takbirs, some of the imams would show negligence in them, hence their mention was made.
Source: Sunan Nasa'i: Translation and Benefits by Shaykh Hafiz Muhammad Amin Hafizullah, Page: 1180