Sahih Ibn Khuzaymah

The imam should not need to observe the actions of the followers or look at any one of them so that he may instruct them to do something or prohibit them from something through such gestures or indications that they understand what action they must perform in their prayer and what they must refrain from.

Sayyiduna Jabir (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings of Allah be upon him) became ill, so we prayed behind him (standing) while he (peace be upon him) was leading the prayer sitting. And Sayyiduna Abu Bakr (may Allah be pleased with him) was saying the takbeer and conveying your (peace be upon him) takbeer to the people. He says that the Prophet (peace be upon him) looked towards us and saw us standing (praying), so he (peace be upon him) gestured to us (to sit), so we sat down. Then when he finished the prayer with salam, he said: "You have just done as the Persians and Romans do. They stand before their kings while they (the kings) are sitting. Therefore, do not do this (in the future). Follow your imams. If the imam leads the prayer standing, then you should pray standing, and if he leads sitting, then you should also pray sitting."

And the hadith of Sayyiduna Sahl bin Hanzaliyah (may Allah be pleased with him), in which it is mentioned that the Prophet (peace and blessings of Allah be upon him) sent Sayyiduna Anas bin Abi Marthad (may Allah be pleased with him) to guard (the mountain pass), he says that the Prophet (peace and blessings of Allah be upon him) kept turning towards the pass during the prayers until, when he completed his prayer, he finished with salam and said to me: "Rejoice, your (guard) horseman has arrived."

Imam Sahib narrates the hadith of Sayyiduna Sahl bin Hanzaliyah (may Allah be pleased with him) from his two respected teachers, Muhammad bin Yahya and Fahd bin Salman.