Sunan At-Tirmidhi

Narrated Abu Ghifar Al-Muthanna bin Sa'eed At-Ta'i:from Abu Tamimah al-Hujaimi from Jabir bin Sulaim who said: ""I went to the Prophet (ﷺ) and I said: ''Alaikas-Salam (upon you be peace)' so he replied: 'Do not say ""'Alaikas-Salam"" rather say As-Salamu Alaik.'"" And he mentioned the story in its entirety