(28) باب مَا جَاءَ فِي كَرَاهِيَةِ أَنْ يَقُولَ عَلَيْكَ السَّلاَمُ مُبْتَدِئًا
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.
Reference: Jami` at-Tirmidhi 2722
In-book reference: Book 42, Hadith 35
English translation: Vol. 5, Book 40, Hadith 2722