Bismillah
The opinion of Sidi Ahmad Dardir (rahimahullah) and he said:
عشر: رد مقتد على إمامه مشيرا له بقلبه لا برأسه، ثم يسن رده على يساره وبه أحد أي من المأمومين أدرك ركعة مع إمامه ولو صبيا
“And the eleventh (Sunnah) is that a follower (the person praying behind an Imam) reply toward the Imam (i.e, say salam in return) in his heart and without turning his head towards the Imam, and it’s also Sunnah to return the Salam to the left if there’s someone on the left who prayed at least a cycle of prayer (Rak’at) in the congregation (Jama’at), even if he was a child.”
Hadiths:
#1 Yahya related to me from Malik from Nafi that Abdullah ibn Umar used to say the tashahhud saying, “In the name of Allah. Greetings belong to Allah. Prayers belong to Allah. Pure actions belong to Allah. Peace be on the Prophet and the mercy of Allah and His blessings. Peace be on us and on the slaves of Allah who are salihun. I testify that there is no god except Allah. I testify that Muhammad is the Messenger of Allah.”
He used to say this after the first two rakas and he would make supplication with whatever seemed fit to him when the tashahhud was completed. When he sat at the end of the prayer, he did the tashahhud in a similar manner, except that after the tashahhud he made supplication with whatever seemed fit to him. When he had completed the tashahhud and intended to say the taslim, he said, “Peace be on the Prophet and His mercy and blessings. Peace be upon us and on the slaves of Allah who are salihun.”
He then said, “Peace be upon you” to his right, and would return the greeting to the imam, and if anyone said “Peace be upon you” from his left he would return the greeting to him.
#2 Yahya related to me from Malik from Yahya ibn Said al-Ansari that al-Qasim ibn Muhammad ibn Muhammad told him that A’isha, the wife of the prophet, may Allah bless him and grant him peace, used to say in the tashahhud,
“Greetings, good words, prayers, pure actions belong to Allah. I testify that there is no god except Allah, alone without partner, and I testify that Muhammad is the slave of Allah and His Messenger. Peace be upon you, Prophet, and the mercy of Allah and His blessings. Peace be upon us and on the slaves of Allah who are salihun. Peace be upon you.”
Hadith source: Imam Malik’s al-Muwatta.