When one says Salam without Alaykum, does the receiving person of the greeting have to respond with Alaykum Salam or is just saying Salam sufficient?

Salam alaikum
When one says Salam without Alaykum, does the receiving person of the greeting have to respond with Alaykum Salam or is just saying Salam sufficient?
Sayyed Mohammad Al-Musawi, Sayyed Mohammad al-Musawi is originally from Iraq and heads up the World Ahlul Bayt Islamic League in London. Other than being involved in various humanitarian projects, he frequently responds to... Answered 3 years ago

It is obligatory to respond to the Salam as it was given, and it is recommended to  add on it to make it a better response. If you were been greeted by (Salam), you must respond with (Salam) as minimum response, but adding Alaykum on Salam is better.

Wassalam.